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Registration No. 333-194050

As filed with the Securities and Exchange Commission on April 2, 2014

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

Amendment No. 1 to

FORM S-4

REGISTRATION STATEMENT

UNDER THE SECURITIES ACT OF 1933

First Interstate BancSystem, Inc.

(Exact name of registrant as specified in its charter)

401 North 31st Street

Billings, Montana 59101

(406) 255-5390

(Address, including zip code, and telephone number, including area code, of registrant’s principal executive offices)

Kevin P. Riley

Executive Vice President and Chief Financial Officer

401 North 31st Street

Billings, Montana 59101

(406) 255-5390

(Name, address, including zip code, and telephone number, including area code, of agent for service)

Approximate date of commencement of proposed sale of the securities to the public:

As soon as practicable after the effective date of this registration statement.

If the securities being registered on this Form are being offered in connection with the formation of a holding company and there is compliance with General Instruction G, check the following box: o

If this Form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering. o

If this Form is a post-effective amendment filed pursuant to Rule 462(d) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering. o

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, or a smaller reporting company. See the definitions of “large accelerated filer,” “accelerated filer” and “smaller reporting company” in Rule 12b-2 of the Exchange Act.

If applicable, place an X in the box to designate the appropriate rule provision relied upon in conducting this transaction:

The Registrant hereby amends this Registration Statement on such date or dates as may be necessary to delay its effective date until the Registrant shall file a further amendment which specifically states that this Registration Statement shall thereafter become effective in accordance with Section 8(a) of the Securities Act of 1933, as amended, or until this Registration Statement shall become effective on such date as the Securities and Exchange Commission, acting pursuant to said Section 8(a), may determine.

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Information contained herein is subject to completion or amendment. A registration statement relating to the shares of First Interstate BancSystem, Inc. Class A Common Stock to be issued in the merger has been filed with the Securities and Exchange Commission. These securities may not be sold nor may offers to buy be accepted prior to the time the registration statement becomes effective. This proxy statement/prospectus shall not constitute an offer to sell or the solicitation of an offer to buy these securities nor shall there be any sale of these securities in any jurisdiction in which such offer, solicitation or sale is not permitted or would be unlawful prior to registration or qualification under the securities laws of any such jurisdiction.

PROXY STATEMENT/PROSPECTUS
DATED APRIL 2, 2014, SUBJECT TO COMPLETION

MOUNTAIN WEST FINANCIAL CORP.

To the Stockholders of Mountain West Financial Corp.:

We are furnishing this proxy statement/prospectus to the holders of common stock of Mountain West Financial Corp., which we refer to as Mountain West, in connection with the solicitation of proxies for a meeting of the stockholders of Mountain West concerning the proposals described in this document.  On February 10, 2014, First Interstate BancSystem, Inc., which we refer to as First Interstate, entered into a merger agreement to acquire Mountain West, in a transaction in which the consideration will be paid partly in stock and partly in cash. If the merger agreement is approved and the merger is subsequently completed, Mountain West will merge with and into First Interstate, with First Interstate as the surviving entity.  The boards of Mountain West and First Interstate have each unanimously approved the merger agreement.  Furthermore, all current directors and certain senior executive officers (and their affiliates) of Mountain West, who held approximately 53.6% of the issued and outstanding shares of Mountain West common stock on the record date, have signed agreements to support the merger and vote in favor of it at the Mountain West stockholder meeting.

Under the merger agreement, at the effective time of the merger, each issued and outstanding share of Mountain West common stock will be cancelled and converted into the right to receive 0.2552 shares of First Interstate Class A Common Stock plus $7.125 in cash or, if a Mountain West stockholder properly elects and subject to the limitations summarized below, an amount in all cash or all stock roughly equal in value to the above described combination of stock and cash merger consideration. Based on the closing price of First Interstate Class A Common Stock on the NASDAQ Global Select Market on March 31, 2014 of $28.22 per share, and on the number of shares and options to purchase shares of Mountain West common stock issued and outstanding on such date, the aggregate value of the consideration that would be expected to be delivered to Mountain West stockholders in the merger would be approximately $78.8 million, of which $39.9 million would be paid in cash and $38.9 million would be represented by 1,378,230 shares of First Interstate Class A Common Stock. On a per share basis (and again based on the March 31, 2014 closing price of First Interstate Class A Common Stock), the value of the consideration that would be delivered to Mountain West stockholders in the meger would be $14.33 per share.  The actual value received by Mountain West stockholders in the aggregate and on a per share of Mountain West common stock basis will fluctuate prior to the completion of the merger based on the prevailing market price of the First Interstate Class A Common Stock. You may obtain current stock price quotations for First Interstate Class A Common Stock on the NASDAQ Global Select Market under the symbol “FIBK.”  Mountain West common stock is not listed on a national securities exchange, and therefore current Mountain West stock price quotations are not available.

As an accommodation to Mountain West stockholders, First Interstate has agreed to allow them to elect to receive their individual merger consideration in either all cash or all First Interstate Class A Common Stock, subject to the limitations discussed below and in lieu of the stock and cash combination merger consideration.  Electing Mountain West stockholders may not ultimately receive in the merger only cash or only stock, however, because the merger agreement provides that First Interstate is only required to issue that number of shares of First Interstate Class A Common Stock that would have been issued in the merger if all Mountain West shares were converted at the specified stock and cash combination merger consideration, which we refer to as the target share number.  If, after giving effect to the merger consideration with respect to “non-electing” Mountain West stockholders, the elections received would result in the issuance of shares of First Interstate Class A Common Stock at the target share number, then the Mountain West shares shall be converted in accordance with the elections.  If, after giving effect to the merger consideration with respect to “non-electing” Mountain West stockholders, the elections received

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would result in the issuance of shares of First Interstate Class A Common Stock in excess of the target share number, then the number of shares of First Interstate Class A Common Stock to be delivered to the Mountain West stockholders making the all stock elections will be reduced proportionately and cash in an equivalent value will be paid to those stockholders.  If, after giving effect to the merger consideration with respect to “non-electing” Mountain West stockholders, the elections received would result in the issuance of fewer shares of First Interstate Class A Common Stock than the target share number, then the amount of cash to be delivered to the Mountain West stockholders making the all cash elections will be reduced proportionately and shares of First Interstate Class A Common Stock having an equivalent value will be issued to those stockholders.

No fractional shares of First Interstate Class A Common Stock will be issued in the merger.  A Mountain West stockholder will receive cash in lieu of any fractional shares of First Interstate Class A Common Stock such holder would otherwise have been entitled to receive in the merger.

Mountain West will hold a special meeting of stockholders to consider the proposed merger and related matters. First Interstate and Mountain West cannot complete the proposed merger unless Mountain West’s stockholders vote to approve the merger agreement and the merger. This letter and the accompanying documents are being provided by the Mountain West board of directors to solicit your proxy to vote for approval of the merger agreement and the merger at a special meeting of Mountain West stockholders. The vote of the stockholders of First Interstate is not required to approve the merger agreement or merger.  This letter and the accompanying documents are also being delivered to Mountain West stockholders as First Interstate’s prospectus for its offering of First Interstate Class A Common Stock in the merger.

We expect the merger to be generally tax free to Mountain West stockholders for United States federal income tax purposes, except with respect to the taxes attributable to the cash to be received by Mountain West stockholders.  Mountain West stockholders are urged to consult your own tax advisors to determine the extent to which your receipt of the merger consideration may be taxable to you.

Your vote is very important. To ensure your representation at the Mountain West special meeting, please complete and return the enclosed proxy card or submit your proxy through the Internet. Whether or not you expect to attend the Mountain West special meeting, please promptly vote. Submitting a proxy now will not prevent you from being able to vote in person at the Mountain West special meeting. The Mountain West board of directors has unanimously approved the merger agreement and the transactions contemplated thereby and recommends that you vote “FOR” the approval of the merger agreement and the merger and “FOR” the adjournment of the special meeting if necessary or appropriate.

The accompanying document provides you with additional detailed information about the proposed merger and the merger agreement. It also contains or references information about Mountain West and First Interstate and related matters. You are encouraged to read the accompanying document carefully, including the information incorporated in the document by reference. In particular, you should read the “RISK FACTORS” section beginning on page 19 for a discussion of the risks you should consider in evaluating the proposed merger and how it will affect you.

Sincerely,

Richard G. Anderson

Chairman of the Board

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of the merger agreement or merger, the issuance of the First Interstate Class A Common Stock in connection with the merger or the other transactions described in this document, or passed upon the adequacy or accuracy of the disclosure in this letter or the accompanying document. Any representation to the contrary is a criminal offense.

The securities to be issued in connection with the merger are not savings accounts, deposits or other obligations of any bank or savings association and are not insured by the Federal Deposit Insurance Corporation or any other governmental agency.

This letter and the accompanying documents are dated April 2, 2014, and are first being mailed to stockholders of Mountain West on or about April 14, 2014.

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MOUNTAIN WEST FINANCIAL CORP.
1225 Cedar Street

Helena, MT 59601

NOTICE OF THE SPECIAL MEETING OF STOCKHOLDERS
TO BE HELD ON May 13, 2014

NOTICE IS HEREBY GIVEN that a special meeting of the stockholders of Mountain West Financial Corp., which we refer to as Mountain West, will be held at the Holiday Inn — Downtown, 22 N. Last Chance Gulch, Helena, MT 59601, at 10:00 a.m., Mountain Time, on May 13, 2014, for the following purposes:

1. To approve the Agreement and Plan of Merger, which we refer to as the merger agreement, dated as of February 10, 2014, by and between First Interstate BancSystem, Inc. and Mountain West, as such merger agreement may be amended from time to time, a copy of which is attached as Appendix A, and the merger to be effected pursuant thereto, which we refer to collectively as the Mountain West Merger proposal; and

2. To approve one or more adjournments of the Mountain West special meeting, if necessary or appropriate, including adjournments to permit further solicitation of proxies in favor of the Mountain West Merger proposal, which we refer to as the Mountain West Adjournment proposal.

Mountain West will transact no other business at the special meeting, except for business properly brought before the special meeting or any adjournment or postponement thereof.

The Mountain West Merger proposal is described in more detail in this document, which you should read carefully in its entirety before you vote. A copy of the merger agreement is attached as Appendix A to this document.

The Mountain West board of directors has set March 31, 2014 as the record date for the Mountain West special meeting. Only holders of record of Mountain West common stock at the close of business on March 31, 2014 will be entitled to notice of and to vote at the Mountain West special meeting and any adjournments or postponements thereof. Any stockholder entitled to attend and vote at the Mountain West special meeting is entitled to appoint a proxy to attend and vote on such stockholder’s behalf. Such proxy need not be a holder of Mountain West common stock.

Your vote is very important. To ensure your representation at the Mountain West special meeting, please complete and return the enclosed proxy card or submit your proxy through the Internet. Please promptly vote whether or not you expect to attend the Mountain West special meeting. Submitting a proxy now will not prevent you from being able to vote in person at the Mountain West special meeting.

The Mountain West board of directors has unanimously approved the merger agreement and the transactions contemplated thereby and recommends that you vote “FOR” the Mountain West Merger proposal and “FOR” the Mountain West Adjournment proposal.

By order of the Board of Directors,

Richard G. Anderson
Chairman of the Board

Helena, Montana

April 2, 2014

PLEASE VOTE YOUR SHARES OF MOUNTAIN WEST COMMON STOCK PROMPTLY. YOU CAN FIND INSTRUCTIONS FOR VOTING ON THE ENCLOSED PROXY CARD. IF YOU HAVE QUESTIONS ABOUT THE PROPOSALS OR ABOUT VOTING YOUR SHARES, PLEASE CALL MOUNTAIN WEST INVESTOR RELATIONS AT 406-449-2265.

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ABOUT THIS DOCUMENT

This document, which forms part of a registration statement on Form S-4 filed with the Securities and Exchange Commission by First Interstate (File No. 333-194050), constitutes a prospectus of First Interstate under Section 5 of the Securities Act of 1933, as amended, which is referred to herein as the Securities Act, with respect to the shares of First Interstate Class A Common Stock to be issued in the merger contemplated by the merger agreement and the transactions contemplated thereby.

This document also constitutes a notice of meeting and a proxy statement with respect to Mountain West’s special meeting, at which Mountain West stockholders will be asked to consider and vote upon certain proposals, including a proposal to approve the merger and the merger agreement and the transactions contemplated thereby.

WHERE YOU CAN FIND MORE INFORMATION

First Interstate files annual, quarterly and current reports, proxy statements and other business and financial information with the Securities and Exchange Commission, which referred to herein as the SEC. You may read and copy any materials that First Interstate files with the SEC at the SEC’s Public Reference Room at 100 F Street, N.E., Room 1580, Washington, D.C. 20549, at prescribed rates. Please call the SEC at (800) SEC-0330 ((800) 732-0330) for further information on the public reference room. In addition, First Interstate files its reports and other business and financial information with the SEC electronically, and the SEC maintains a website located at http://www.sec.gov containing this information. You are also able to obtain these documents, free of charge, from First Interstate at www.FIBK.com under the tab “SEC Filings.” The information contained on or accessible from First Interstate’s website does not constitute a part of this document and is not incorporated by reference herein.

First Interstate has filed a registration statement on Form S-4 of which this document forms a part. As permitted by SEC rules, this document does not contain all of the information included in the registration statement or in the exhibits or schedules to the registration statement. You may read and copy the registration statement, including any amendments, schedules and exhibits at the address set forth below. Statements contained in this document as to the contents of any contract or other documents referred to in this document are not necessarily complete. In each case, you should refer to the copy of the applicable contract or other document filed as an exhibit to the registration statement. This document incorporates by reference documents that First Interstate has previously filed and may hereafter file with the SEC. They contain important information about First Interstate and its financial condition. For further information, please see the section entitled “INCORPORATION OF CERTAIN DOCUMENTS BY REFERENCE” beginning on page 79.

First Interstate will provide to each person to whom a copy of this prospectus is delivered, a copy of any or all of the information that it has incorporated by reference into this document (excluding exhibits to such documents unless such exhibits are specifically incorporated by reference). First Interstate will provide this information upon written or oral request at no cost to the requester. You may request this information by contacting First Interstate’s corporate headquarters at the following address: 401 North 31st Street, Billings, Montana 59101, Attention: Amy Anderson, or by calling (406) 255-5390.

To obtain timely delivery of these documents, you must request the information no later than May 5, 2014 in order to receive them before Mountain West’s special meeting of stockholders.

First Interstate Class A Common Stock is traded on the NASDAQ Global Select Market under the symbol “FIBK.”

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QUESTIONS AND ANSWERS ABOUT THE MERGER AND THE SPECIAL MEETING

WHAT IS THE MERGER?

First Interstate and Mountain West have entered into an Agreement and Plan of Merger, which is referred to herein as the merger agreement, pursuant to which Mountain West will merge with and into First Interstate with First Interstate continuing as the surviving corporation in a transaction that is referred to in this proxy statement/prospectus as the merger. A copy of the merger agreement is attached as Appendix A to this document. If the merger is consummated, First Interstate and Mountain West expect that within six months Mountain West Bank, National Association, a wholly owned subsidiary of Mountain West and which is referred to in this proxy statement/prospectus as Mountain West Bank, will merge with and into First Interstate Bank, which is a wholly owned subsidiary of First Interstate.  The merger of Mountain West Bank with and into First Interstate Bank is referred in this proxy statement/prospectus as the bank merger. In order for Mountain West to complete the transaction with First Interstate, the approval of Mountain West’s stockholders is needed as are approvals by the bank regulators of First Interstate, Mountain West, First Interstate Bank, and Mountain West Bank.

WHY AM I RECEIVING THIS PROXY STATEMENT/PROSPECTUS?

Mountain West is sending these materials to its stockholders to help them decide how to vote their shares of Mountain West common stock with respect to the merger and other matters to be considered at the special meeting. More specifically, the merger cannot be completed unless Mountain West stockholders approve the merger agreement. Mountain West is holding a special meeting of its stockholders to vote on the proposals necessary to complete the merger. Information about the special meeting, the merger and the other business to be considered by Mountain West’s stockholders at the special meeting is contained in this document.

In addition, this document constitutes a prospectus of First Interstate. It is a prospectus because First Interstate, in connection with the merger, is offering shares of its Class A Common Stock, in addition to cash consideration, in exchange for outstanding shares of Mountain West common stock in the merger. The shares being offered by this document are required to be registered with the SEC prior to their issuance.

WHAT WILL MOUNTAIN WEST STOCKHOLDERS RECEIVE IN THE MERGER?

Under the merger agreement, First Interstate expects to issue approximately 1,378,230 shares of its Class A Common Stock and to pay approximately $39.9 million in cash for all of the outstanding shares of common stock of Mountain West, based on the 5,660,748 shares of Mountain West common stock and options to purchase shares of Mountain West common stock outstanding as of March 31, 2014, which is referred to in this proxy statement/prospectus as the record date.

WHAT WILL EACH MOUNTAIN WEST STOCKHOLDER RECEIVE IN THE MERGER?

Each Mountain West stockholder will receive a combination of cash and stock consisting of 0.2552 shares of First Interstate Class A Common Stock plus $7.125 in cash per share of Mountain West common stock; provided that, if a stockholder chooses, the stockholder may elect to receive one of the following forms of merger consideration instead of such cash and stock combination

·                       all cash, in an amount per share of Mountain West common stock equal, subject to proration and adjustment as described herein, to (i) $7.125 plus (ii) the product of (x) 0.2552 and (y) the average closing price of First Interstate Class A Common Stock on the NASDAQ Global Select Market during the twenty (20) business days immediately preceding the fifth day prior to the closing date of the merger, which is referred to herein as the average closing price; or

·                       all First Interstate Class A Common Stock, in an amount of shares of First Interstate Class A Common Stock per share of Mountain West common stock equal, subject to proration and adjustment as described herein, to (i) 0.2552 plus (ii) the quotient of (x) $7.125 divided by (y) the average closing price.

All elections are subject to the election, proration and allocation procedures and limitations described in this proxy statement/prospectus.  Due to these limitations, Mountain West stockholders electing all cash or all stock will be subject to a pro rata adjustment as may be necessary, because the merger agreement provides that First Interstate is only required to issue that number of shares of First Interstate Class A Common Stock that would have been issued in

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the merger if all Mountain West shares were converted at the specified stock and cash combination merger consideration (referred to herein as the target share number).

HOW AND WHEN DOES A MOUNTAIN WEST STOCKHOLDER ELECT THE FORM OF CONSIDERATION HE OR SHE PREFERS TO RECEIVE?

If a Mountain West stockholder desires to make an all cash or all stock election (rather than receive the established merger consideration consisting of a mix of cash and stock, for which no election need be made), a Mountain West stockholder must submit an election statement (which accompanies this proxy statement/prospectus) to the exchange agent before 5:00 p.m., Mountain Time, on the date that is five business days prior to the closing date of the merger (or such other time and date as First Interstate and Mountain West may mutually agree upon). This date is referred to as the election deadline. Election choices and election procedures are described under the caption entitled “THE MERGER.”

NOTE: The actual election deadline is not currently known. First Interstate and Mountain West will issue a press release announcing the date of the election deadline at least five business days prior to the election deadline. Additionally, First Interstate and Mountain West will post the date of the election deadline on their respective web sites, also at least five business days before that deadline.

MAY A MOUNTAIN WEST STOCKHOLDER CHANGE HIS OR HER ELECTION ONCE IT HAS BEEN SUBMITTED?

Yes. An election may be revoked or changed so long as the new election is received by the exchange agent at or prior to the election deadline. The exchange agent has reasonable discretion to determine whether any election, revocation or change has been timely made.

WHAT HAPPENS IF AN ELECTION IS NOT MADE PRIOR TO THE ELECTION DEADLINE?

If a Mountain West stockholder chooses not to submit an election statement to the exchange agent prior to the election deadline, then that holder will receive, for each share of Mountain West common stock owned by such holder, 0.2552 shares of First Interstate Class A Common Stock plus $7.125 in cash.

WILL FRACTIONAL SHARES OF FIRST INTERSTATE CLASS A COMMON STOCK BE ISSUED IN THE MERGER?

No. A Mountain West stockholder will receive cash in lieu of any fractional shares of First Interstate Class A Common Stock such holder would otherwise have been entitled to receive in the merger.

WHAT HAPPENS TO MOUNTAIN WEST STOCK OPTIONS IN THE MERGER?

Under the merger agreement, Mountain West has agreed that every option to purchase Mountain West common stock outstanding and not expired immediately prior to the completion of the merger, whether or not then vested and whether or not then exercisable, shall be cancelled in exchange for a payment in cash equal to (x) the per share merger consideration value, minus (y) the applicable exercise price per share, multiplied by (z) the number of shares available under such option, assuming 100% vesting.

WHAT ARE THE MATERIAL UNITED STATES FEDERAL INCOME TAX CONSEQUENCES OF THE MERGER TO MOUNTAIN WEST STOCKHOLDERS?

The merger is intended to qualify, and the obligation of First Interstate to complete the merger is conditioned upon the receipt of a legal opinion from its counsel to the effect that the merger will qualify, as a reorganization within the meaning of Section 368 of the Internal Revenue Code of 1986, as amended, which is referred to herein as the Internal Revenue Code.  In addition, in connection with the filing of the registration statement of which this document is a part, Holland & Hart LLP has delivered an opinion to First Interstate to the same effect.

Assuming the merger qualifies as such a reorganization, a stockholder of Mountain West generally will not recognize any gain or loss upon receipt of First Interstate Class A Common Stock in exchange for Mountain West common stock in the merger but may recognize gain with respect to the cash consideration and cash received in lieu of a fractional share of First Interstate Class A Common Stock.

The consequences of the merger to any particular Mountain West stockholder will depend on that stockholder’s particular facts and circumstances. Accordingly, you are urged to consult your tax advisor to determine your tax consequences from the merger and your election statement.

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WHEN WILL THE MERGER BE COMPLETED?

First Interstate and Mountain West are working to complete the merger as soon as practicable. If the stockholders of Mountain West approve the merger agreement and the merger, the parties currently expect that the merger will be completed in mid- 2014, but no later than December 31, 2014.  Neither First Interstate nor Mountain West can predict, however, the actual date on which the merger will be completed because it is subject to factors beyond each company’s control, including whether or when the required regulatory approvals will be received.

WHEN AND WHERE WILL THE MOUNTAIN WEST SPECIAL MEETING OF STOCKHOLDERS TAKE PLACE?

The special meeting of stockholders of Mountain West will be held at 10:00 a.m., Mountain Time, on May 13, 2014, at the Holiday Inn — Downtown, 22 N. Last Chance Gulch, Helena, MT 59601.

WHAT AM I BEING ASKED TO VOTE ON AND WHY IS THIS APPROVAL NECESSARY?

Mountain West stockholders are being asked to vote on the following proposals:

1. to approve the merger agreement, a copy of which is attached as Appendix A to this document, and the merger to be effected pursuant thereto, which are referred to herein collectively as the Mountain West Merger proposal; and

2. to approve one or more adjournments of the Mountain West special meeting, if necessary or appropriate, including adjournments to permit further solicitation of proxies in favor of the Mountain West Merger proposal, which is referred to as the Mountain West Adjournment proposal.

Approval by Mountain West stockholders of the Mountain West Merger proposal is required for completion of the merger. Mountain West will transact no other business at the Mountain West special meeting, except for business properly brought before the Mountain West special meeting or any adjournment or postponement thereof.

WHO IS ENTITLED TO VOTE?

Holders of record of Mountain West common stock at the close of business on March 31, 2014, which is the date that the Mountain West board of directors has fixed as the record date for the Mountain West special meeting, are entitled to vote at the Mountain West special meeting.

WHAT CONSTITUTES A QUORUM?

The presence at the Mountain West special meeting, in person or by proxy, of holders of a majority of the issued and outstanding shares of Mountain West common stock entitled to vote at the Mountain West special meeting will constitute a quorum for the transaction of business. Abstentions, if any, will be included in determining the number of shares present at the meeting for the purpose of determining the presence of a quorum. So-called broker non-votes, which are described below and not anticipated at this meeting, will have no impact on the determination of a quorum.

WHAT VOTE IS REQUIRED TO APPROVE EACH PROPOSAL AT THE MOUNTAIN WEST SPECIAL MEETING?

Mountain West Merger proposal: The affirmative vote of two-thirds of the outstanding shares of Mountain West common stock entitled to vote is required to approve the Mountain West Merger proposal. As described in this proxy statement, all current directors and certain senior executive officers (and their affiliates) of Mountain West have signed agreements to vote in favor of and support the merger. Such persons own approximately 53.6% of outstanding Mountain West common stock as of the record date.

Mountain West Adjournment proposal: Assuming a quorum is present, the affirmative vote of a majority of the shares of Mountain West common stock represented (in person or by proxy) at the Mountain West special meeting and entitled to vote on the proposal is required to approve the Mountain West adjournment proposal.

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WHAT DOES THE MOUNTAIN WEST BOARD OF DIRECTORS RECOMMEND?

The Mountain West board of directors unanimously recommends that Mountain West stockholders vote “FOR” the Mountain West Merger proposal and “FOR” the Mountain West Adjournment proposal.

WHAT DO I NEED TO DO NOW?

After carefully reading and considering the information contained in this proxy statement/prospectus, if you were the record holder of shares of Mountain West common stock as of March 31, 2014, you may vote in person by attending the special meeting or, to ensure that your shares of Mountain West common stock are represented at the special meeting, you may vote your shares over the Internet or by signing and returning the enclosed proxy card in the postage-paid envelope provided. Please follow the instructions set forth on the proxy card or on the voting instruction form provided by the record holder if your shares are held in the name of your broker, bank or other nominee.

HOW DO I VOTE?

If you are a stockholder of record of Mountain West as of March 31, 2014, which is referred to as the Mountain West record date, you may submit your proxy before Mountain West’s special meeting in one of the following ways:

·                  visit the website shown on your proxy card to vote via the Internet; or

·                  complete, sign, date and return the enclosed proxy card in the enclosed postage-paid envelope.

You may also cast your vote in person at Mountain West’s special meeting.

If your shares are held in “street name,” through a broker, bank or other nominee, that institution will send you separate instructions describing the procedure for voting your shares. “Street name” stockholders who wish to vote at the meeting will need to obtain a proxy form from their broker, bank or other nominee.

HOW MANY VOTES DO I HAVE AND WHAT PERCENTAGE OF SHARES DO DIRECTORS AND OFFICERS OWN?

You are entitled to one vote for each share of Mountain West common stock that you owned as of the record date. As of the close of business on the record date, there were 5,400,587 shares of Mountain West common stock issued and outstanding. As of that date, approximately 53.6% of the outstanding shares of Mountain West common stock were beneficially owned by the current directors and certain senior executive officers (and their affiliates) of Mountain West.

IF MY SHARES ARE HELD IN “STREET NAME” BY A BROKER, BANK OR OTHER NOMINEE, WILL MY BROKER, BANK OR OTHER NOMINEE VOTE MY SHARES FOR ME?

If your shares are held in “street name” in a stock brokerage account or by a bank or other nominee, you must provide the record holder of your shares with instructions on how to vote your shares. Please follow the voting instructions provided by your broker, bank or other nominee. Please note that you may not vote shares held in street name by returning a proxy card directly to Mountain West or by voting in person at Mountain West’s special meeting unless you provide a “legal proxy,” which you must obtain from your broker, bank or other nominee.

Under the rules of the NASDAQ Stock Market, brokers who hold shares in street name for a beneficial owner of those shares typically have the authority to vote in their discretion on “routine” proposals when they have not received instructions from beneficial owners. However, brokers are not allowed to exercise their voting discretion with respect to the approval of matters that the NASDAQ determines to be “non-routine” without specific instructions from the beneficial owner. It is expected that all proposals to be voted on at the Mountain West special meeting are such “non-routine” matters. Broker non-votes occur when a broker or nominee is not instructed by the beneficial owner of shares to vote on a particular proposal for which the broker does not have discretionary voting power.

Assuming a quorum is present, if you are a Mountain West stockholder and you do not instruct your broker, bank or other nominee on how to vote your shares,

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·                  your broker, bank or other nominee may not vote your shares on the Mountain West Merger proposal, which broker non-votes will have the same effect as a vote “AGAINST” such proposal; and

·                  your broker, bank or other nominee may not vote your shares on the Mountain West Adjournment proposal, which broker non-votes will have no effect on the vote count for such proposal.

WHAT IF I DO NOT VOTE OR ABSTAIN?

Assuming a quorum is present, an abstention occurs when a stockholder attends the special meeting in person and does not vote or returns a proxy with an “abstain” vote.

Assuming a quorum is present, if you are a Mountain West stockholder and you fail to vote or fail to instruct your broker, bank or other nominee how to vote on the Mountain West Merger proposal, it will have the same effect as a vote cast “AGAINST” the Mountain West Merger proposal. If you respond with an “abstain” vote on the Mountain West Merger proposal, your proxy will have the same effect as a vote cast “AGAINST” the Mountain West Merger proposal.

WHAT WILL HAPPEN IF I RETURN MY PROXY OR VOTING INSTRUCTION CARD WITHOUT INDICATING HOW TO VOTE?

If you sign and return your proxy or voting instruction card without indicating how to vote on any particular proposal, the Mountain West common stock represented by your proxy will be voted as recommended by the Mountain West board of directors with respect to that proposal. Unless a Mountain West stockholder, as applicable, checks the box on its proxy card to withhold discretionary authority, the proxyholders may use their discretion to vote on other matters that may properly come before the meeting, if any the Mountain West special meeting.

MAY I CHANGE MY VOTE AFTER I HAVE DELIVERED MY PROXY OR VOTING INSTRUCTION CARD?

Yes. You may change your vote at any time before your proxy is voted at the Mountain West special meeting. You may do this in one of four ways:

·                  by sending a notice of revocation to the corporate secretary of Mountain West;

·                  by logging onto the Internet website specified on your proxy card in the same manner you would to submit your proxy electronically if you are eligible to do so and following the prompts;

·                  by sending a completed proxy card bearing a later date than your original proxy card; or

·                  by attending the Mountain West special meeting and voting in person.

If you choose any of the first three methods, you must take the described action such that the notice, internet vote or proxy card, as applicable, is received no later than the beginning of the special meeting.

If your shares are held in an account at a broker, bank or other nominee, you should contact your broker, bank or other nominee to change your vote.

DO I NEED IDENTIFICATION TO ATTEND THE MOUNTAIN WEST MEETING IN PERSON?

Yes. Please bring valid picture identification. In addition, if your shares are held in street name, please bring acceptable proof of ownership, such as a letter from your broker or an account statement showing that you beneficially owned shares of Mountain West common stock, as applicable, on the record date.

ARE MOUNTAIN WEST STOCKHOLDERS ENTITLED TO DISSENTERS’ RIGHTS?

Under Montana law, Mountain West stockholders are entitled to dissenters’ rights in connection with the merger.

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WHAT HAPPENS IF THE MERGER IS NOT COMPLETED?

If the merger is not completed, Mountain West stockholders will not receive any consideration for their shares of Mountain West common stock in connection with the merger. Instead, Mountain West will remain an independent private company and its common stock will continue to be unlisted. Under specified circumstances described elsewhere herein, Mountain West may be required to pay First Interstate a fee in connection with the termination of the merger agreement.

SHOULD MOUNTAIN WEST STOCKHOLDERS SEND IN THEIR STOCK CERTIFICATES NOW?

No. Mountain West stockholders SHOULD NOT send in any stock certificates now. If the merger is approved, a letter of transmittal with instructions for its completion will be provided to Mountain West stockholders under separate cover and the stock certificates should be sent at that time.

WHOM SHOULD I CONTACT IF I HAVE ANY QUESTIONS ABOUT THE PROXY MATERIALS OR VOTING?

If you are a Mountain West stockholder and have any questions about the proxy materials or if you need assistance submitting your proxy or voting your shares or need additional copies of this document or the enclosed proxy card, you should contact Mountain West Investor Relations at 406-449-2265.

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SUMMARY

This summary highlights selected information included in this document and does not contain all of the information that may be important to you. You should read this entire document and its appendices and the other documents referred to before you decide how to vote with respect to the merger-related proposals. In addition, important business and financial information about First Interstate is incorporated by reference into this document. For a description of this information, please see the section entitled “INCORPORATION OF CERTAIN DOCUMENTS BY REFERENCE” beginning on page 79. You may obtain the information incorporated by reference into this document without charge by following the instructions in the section entitled “WHERE YOU CAN FIND MORE INFORMATION” in the forepart of this document. Each item in this summary includes a page reference directing you to a more complete description of that item.

Unless the context otherwise requires, throughout this document, “First Interstate” refers to First Interstate BancSystem, Inc., “Mountain West” refers to Mountain West Financial Corp. and “the combined company” refers collectively to First Interstate and Mountain West. Also, the proposed merger of Mountain West with and into First Interstate is referred to as, the “merger,” the proposed merger of Mountain West Bank with and into First Interstate Bank as the “bank merger,”  and the Agreement and Plan of Merger, dated as of February 10, 2014, by and between First Interstate and Mountain West as the “merger agreement.”

Information about the Companies (page 26)

First Interstate BancSystem, Inc.
401 North 31st Street
Billings, Montana
Phone: (406) 255-5390

First Interstate is a financial and bank holding company headquartered in Billings, Montana and registered under the Bank Holding Company Act of 1956, as amended, which is referred to herein as the BHC Act. As of December 31, 2013, First Interstate had consolidated assets of $7.6 billion, deposits of $6.1 billion, loans of $4.3 billion and total stockholders’ equity of $802 million. As of the date of this document, First Interstate operates 74 banking offices, including detached drive-up facilities, in 41 communities located in Montana, Wyoming and western South Dakota. Through First Interstate Bank, First Interstate delivers a comprehensive range of banking products and services to individuals, businesses, municipalities and other entities throughout First Interstate’s market areas. First Interstate’s customers participate in a wide variety of industries, including energy, healthcare and professional services, education and governmental services, construction, mining, agriculture, retail and wholesale trade and tourism.

Mountain West Financial Corp.

1225 Cedar Street

Helena, Montana
Phone: (406) 449-2265

Mountain West is a bank holding company headquartered in Helena, Montana and registered under the BHC Act.  As of December 31, 2013, Mountain West had consolidated assets of $646 million, deposits of $540 million, loans of $405 million and total stockholders’ equity of $62 million.  As of the date of this document, Mountain West operates 12 branches in five primary markets in Montana — Helena, Great Falls, Missoula, the Flathead Valley (Kalispell and Whitefish) and Bozeman. Through Mountain West Bank, Mountain West offers a full range of commercial and consumer banking products and services to individuals, businesses and organizations in Montana.

The Merger and the Merger Agreement (pages 27 and 48)

The terms and conditions of the merger are contained in the merger agreement, which is attached to this document as Appendix A. You are encouraged you to read the merger agreement carefully, as it is the legal document that governs the merger.  Under the terms of the merger agreement, Mountain West will merge with and into First Interstate with First Interstate as the surviving corporation.

Merger Consideration (page 27)

Under the merger agreement, at the effective time of the merger, each issued and outstanding share of Mountain West

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common stock will be cancelled and converted into the right to receive 0.2552 shares of First Interstate Class A Common Stock plus $7.125 in cash or, if a Mountain West stockholder properly elects and subject to the limitations summarized below, an amount in all cash or all stock roughly equal in value to the above described combination of stock and cash merger consideration.  Based on the closing price of First Interstate Class A Common Stock on the NASDAQ Global Select Market on March 31, 2014 of $28.22 per share and on the number of shares and options to purchase shares of Mountain West common stock issued and outstanding on such date, the aggregate value of the consideration that would be expected to be delivered to Mountain West stockholders in the merger would be approximately $78.8 million, of which $39.9 million would be paid in cash and $38.9 million would be represented by 1,378,230 shares of First Interstate Class A Common Stock.  On a per share basis (and again based on the March 31, 2014 closing price of First Interstate Class A Common Stock), the value of the consideration that would be delivered to Mountain West stockholders in the merger would be $14.33 per share.  The actual value received by Mountain West stockholders in the aggregate and on a per share of Mountain West common stock basis will fluctuate prior to the completion of the merger based on the prevailing market price of the First Interstate Class A Common Stock. You may obtain current stock price quotations for First Interstate Class A Common Stock on the NASDAQ Global Select Market under the symbol “FIBK.”  Mountain West common stock is not listed on a national securities exchange, and therefore current Mountain West stock price quotations are not available.

As an accommodation to Mountain West stockholders, First Interstate has agreed to allow them to elect to receive their individual merger consideration in either all cash or all First Interstate Class A Common Stock, subject to the limitations discussed below and in lieu of the stock and cash combination merger consideration.  Electing Mountain West stockholders may not ultimately receive in the merger only cash or only stock, however, because the merger agreement provides that First Interstate is only required to issue that number of shares that equals the target share number.  If, after giving effect to the merger consideration with respect to “non-electing” Mountain West stockholders, the elections received would result in the issuance of shares of First Interstate Class A Common Stock at the target share number, then the Mountain West shares shall be converted in accordance with the elections.  If, after giving effect to the merger consideration with respect to “non-electing” Mountain West stockholders, the elections received would result in the issuance of shares of First Interstate Class A Common Stock in excess of the target share number, then the number of shares of First Interstate Class A Common Stock to be delivered to the Mountain West stockholders making the all stock elections will be reduced proportionately and cash in an equivalent value will be paid to those stockholders.  If, after giving effect to the merger consideration with respect to “non-electing” Mountain West stockholders, the elections received would result in the issuance of fewer shares of First Interstate Class A Common Stock than the target share number, then the amount of cash to be delivered to the Mountain West stockholders making the all cash elections will be reduced proportionately and shares of First Interstate Class A Common Stock having an equivalent value will be issued to those stockholders.

Recommendation of the Mountain West Board of Directors and Reasons for the Merger (page 33)

The Mountain West board of directors unanimously recommends that Mountain West stockholders vote “FOR” the Mountain West Merger proposal and “FOR” the Mountain West Adjournment proposal.

For a more complete description of Mountain West’s reasons for the merger and the recommendation of the Mountain West board of directors, please see the section entitled “THE MERGER—Recommendation of the Mountain West Board of Directors and Reasons for the Merger” beginning on page 33.

Mountain West Financial Advisor

On April 2, 2014, D.A. Davidson & Co., which is referred to herein as Davidson, Mountain West’s financial advisor in connection with the merger, delivered a fairness opinion to Mountain West’s board of directors, that, as of such date and subject to and based on the qualifications and assumptions set forth in its written opinion, the per share merger consideration in the proposed merger was fair, from a financial point of view, to the stockholders of Mountain West.  This fairness opinion updated the opinion given to Mountain West’s board of directors on February 10, 2014 that as of such date and subject to and based on the qualifications and assumptions set forth in its written opinion, the per share merger consideration in the proposed merger was fair, from a financial point of view, to the stockholders of Mountain West.

The full text of Davidson’s opinion dated April 2, 2014 is attached as Appendix B to this document. You should read the opinion in its entirety for a description of the procedures followed, assumptions made, matters considered, and qualifications and limitations on the review undertaken by Davidson in rendering its opinion.

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Davidson’s opinion is addressed to Mountain West’s board of directors and the opinion is not a recommendation as to how any stockholder of Mountain West should vote with respect to the merger or any other matter or as to any action that a stockholder should take with respect to the merger.

The opinion addresses only the fairness, from a financial point of view, of the per share merger consideration in the proposed merger to the stockholders of Mountain West, and does not address the underlying business decision of Mountain West to engage in the merger, or the relative merits of the merger as compared to any strategic alternatives that may be available to Mountain West.  Davidson has received a fee for its services.

For further information, please see the section entitled “THE MERGER—Opinion of Mountain West’s Financial Advisor” beginning on page 35.

Mountain West Special Meeting of Stockholders (page 22)

The Mountain West special meeting will be held at 10:00 a.m., Mountain Time, on May 13, 2014, at the Holiday Inn — Downtown, 22 N. Last Chance Gulch, Helena, MT 59601. At the Mountain West special meeting, Mountain West stockholders will be asked to approve the Mountain West Merger proposal and the Mountain West Adjournment proposal.

Mountain West’s board of directors has fixed the close of business on March 31, 2014 as the record date for determining the holders of Mountain West common stock entitled to receive notice of and to vote at the Mountain West special meeting. Only holders of record of Mountain West common stock at the close of business on the Mountain West record date will be entitled to notice of and to vote at the Mountain West special meeting and any adjournment or postponement thereof, unless the adjournment is for more than 30 days, or if after the adjournment a new record date is fixed for the adjourned meeting, a notice of the adjourned meeting shall be given to each stockholder of record entitled to vote at such meeting.

As of the record date, there were 5,400,587 shares of Mountain West common stock outstanding held by approximately 330 holders of record.  Each share of Mountain West common stock entitles the holder to one vote on each proposal to be considered at the Mountain West special meeting. All current directors and certain senior executive officers (and their affiliates) of Mountain West have signed agreements to vote in favor of and support the merger. Such persons own approximately 53.6% of outstanding Mountain West common stock as of the record date.  As of the record date, First Interstate and its executive officers and directors beneficially held no shares of Mountain West’s common stock.

Financial Interests of Directors and Officers of Mountain West in the Merger (page 43)

Certain of Mountain West’s executive officers and directors have financial interests in the merger that are different from, or in addition to, the interests of Mountain West’s stockholders. These interests include:

·                  Mr. Bradshaw is partially vested in a benefit under a Salary Continuation Agreement that ordinarily would provide a stream of payments over 20 years beginning at age 62 (or if later, his retirement).  If Mr. Bradshaw’s employment is terminated within 24 months after the merger, Mr. Bradshaw will be fully vested in the payments, such that he would receive monthly payments over 20 years at an annual rate of $36,000 beginning immediately after his termination of employment.

·                  Mr. Brown is partially vested in a benefit under a Salary Continuation Agreement that ordinarily would provide a stream of payments over 20 years beginning at age 65 (or if later, his retirement).  If Mr. Brown’s employment is terminated within 24 months after the merger, Mr. Brown will instead receive a lump sum payment of $77,600, $159,856, or $246,977 (depending on the date of termination) immediately after his termination of employment.

·                  Pursuant to the terms of his 2002 Employment Agreement, Mr. Morgan will be entitled to a lump sum payment of $532,223 on account of the merger, payable upon the closing of the merger.

·                  Mr. Morgan is fully vested in a benefit under a Salary Continuation Agreement that ordinarily would provide a stream of payments over 20 years beginning at his retirement; however, if Mr. Morgan’s employment is terminated within 24 months after the merger, Mr. Morgan would instead receive a lump sum amount equal to $777,515 at the time of his termination of employment.

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·                  The directors and officers of Mountain West and Mountain West Bank will receive continued indemnification and director and officer liability insurance coverage for a period of six years after completion of the merger.

Treatment of Mountain West Stock Options (page 31)

Under the merger agreement, Mountain West has agreed that every option to purchase Mountain West common stock outstanding and not expired immediately prior to the completion of the merger, whether or not then vested and whether or not then exercisable, shall be cancelled in exchange for a payment in cash equal to (x) the per share merger consideration value, minus (y) the applicable exercise price per share, multiplied by (z) the number of shares available under such option, assuming 100% vesting.

Regulatory Approvals Required for the Merger (page 45)

Completion of the merger and the bank merger are subject to various regulatory approvals, including the Board of Governors of the Federal Reserve System, which is referred to herein as Federal Reserve, and the Montana Division of Banking and Financial Institutions. Notifications and/or applications requesting approval for the merger or for the bank merger may also be submitted to other federal and state regulatory authorities. First Interstate and Mountain West have filed, or are in the process of filing, notices and applications to obtain the necessary regulatory approvals. Although First Interstate and Mountain West currently believe they should be able to obtain all required regulatory approvals in a timely manner, they cannot be certain when or if they will obtain them or, if obtained, whether they will contain terms, conditions or restrictions not currently contemplated that will be detrimental to or have a material adverse effect on First Interstate after the completion of the merger. The regulatory approvals to which completion of the merger and bank merger are subject are described in more detail under the section entitled “THE MERGER—Regulatory Approvals Required for the Mergers” beginning on page 46.  Regulatory approvals are also required for Mountain West to pay those dividends to its stockholders that it is permitted to pay pursuant to the merger agreement.

Conditions to the Merger (page 57)

Among other things, the obligations of First Interstate and Mountain West to complete the merger are each subject to the satisfaction or waiver of the following conditions:

·                  approval of the Mountain West Merger proposal by the Mountain West stockholders;

·                  the effectiveness of the registration statement on Form S-4, of which this document is a part, and the absence of a stop order or proceeding initiated or threatened by the SEC for that purpose;

·                  the accuracy of the representations and warranties of each party as of the closing date of the merger;

·                  performance in all material respects by each party of the obligations required to be performed by it at or prior to the closing date of the merger;

·                  receipt of all consents and approvals required to consummate the merger; and

·                  receipt by First Interstate of an opinion of its tax counsel as to certain tax matters.

No Solicitation and Change in Recommendation (page 52)

Under the terms of the merger agreement, Mountain West has agreed not to solicit, initiate, encourage or facilitate inquiries or proposals with respect to, participate in any discussions or negotiations concerning, or enter into any agreement regarding any acquisition proposal. Notwithstanding these restrictions, the merger agreement provides that, under specified circumstances and with advance notice to First Interstate, in response to an unsolicited bona fide acquisition proposal which, in the good faith judgment of the Mountain West board of directors, is or is reasonably likely to result in a proposal which is superior to the merger with First Interstate, and the Mountain West board of directors determines in good faith (and after consultation with Mountain West’s outside counsel) that failure to take such actions would reasonably be expected to be a violation of its fiduciary duties under applicable law, Mountain West may furnish information regarding Mountain West and participate in discussions and negotiations with such third party.

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Termination of the Merger Agreement (page 53)

First Interstate and Mountain West may mutually agree at any time to terminate the merger agreement without completing the merger, even if the Mountain West stockholders have voted to approve the merger agreement.  The merger agreement may also be terminated and the merger abandoned at any time prior to the effective time of the merger, as follows:

·                  by either First Interstate or Mountain West, if the merger is not completed on or before December 31, 2014, unless the failure of the closing to occur by such date is due to a breach of a representation, warranty, covenant or agreement by the party seeking to terminate the merger agreement;

·                  by Mountain West, if the average closing price of First Interstate’s Class A Common Stock on the NASDAQ Global Market is less than $20.00 per share during the twenty (20) business days preceding the fifth business day prior to the closing of the merger, which is referred to herein as the average closing price, unless First Interstate, within five (5) business days of receiving a notice of termination by Mountain West, adjusts the merger consideration by an amount equal to the difference between $20.00 and the average closing price, multiplied by the number of shares of First Interstate Class A Common Stock to be issued in the merger;

·                  by either First Interstate or Mountain West, if a required regulatory approval is denied and First Interstate elects not to appeal such denial within fifteen (15) business days of such denial;

·                  by either First Interstate or Mountain West, if the stockholders of Mountain West have not approved the merger at the Mountain West special meeting or any adjournment or postponement thereof;

·                  by either First Interstate or Mountain West, as applicable, if the closing conditions of the other party are not satisfied or waived or become impossible to satisfy (other than as a result of the terminating party to comply with its obligations under the merger agreement) as of December 31, 2014;

·                  by First Interstate, if environmental site assessments indicate a material adverse environmental condition that First Interstate reasonably determines would have a material adverse effect on Mountain West on a consolidated basis and Mountain West or its subsidiary is unable to remedy the condition within sixty (60) days after First Interstate delivers notice thereof;

·                  subject to certain limitations, by Mountain West, if Mountain West’s board of directors determines an acquisition proposal is a superior proposal and delivers a break-up fee equal to $3 million by wire transfer of immediately available funds to First Interstate; or

·                  by either First Interstate or Mountain West, as applicable, if the other party breaches any of its representations, warranties, agreements or covenants set forth in the merger agreement, which breach is not, or cannot be, cured within thirty (30) days following notice to the breaching party.

Mountain West may be required to pay First Interstate a termination fee equal to $1 million or a break-up fee equal to $3 million in certain circumstances. For more information, please see the section entitled “THE MERGER AGREEMENT — Termination of Merger Agreement —Termination Fee and Break-up Fee” beginning on page 54.

Material United States Federal Income Tax Consequences of the Merger (page 62)

The merger is intended to qualify as a reorganization within the meaning of Section 368 of the Internal Revenue Code. Assuming the merger qualifies as such a reorganization, a stockholder of Mountain West generally will not recognize any gain or loss upon receipt of First Interstate Class A Common Stock in exchange for Mountain West common stock in the merger, but may recognize gain with respect to the cash consideration and cash received in lieu of a fractional share of First Interstate Class A Common Stock. It is a condition to the completion of the merger that First Interstate receives a written opinion from its counsel to the effect that the merger will qualify as a reorganization within the meaning of Section 368 of the Internal Revenue Code.

Tax matters are very complicated and the tax consequences of the merger to each Mountain West stockholder may depend on such stockholder’s particular facts and circumstances. Mountain West stockholders are urged to consult their tax

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advisors to understand fully the tax consequences to them of the merger. For more information, please see the section entitled “MATERIAL UNITED STATES FEDERAL INCOME TAX CONSEQUENCES OF THE MERGER” beginning on page 62.

Comparison of Stockholders’ Rights (page 73)

The rights of Mountain West stockholders who continue as First Interstate stockholders after the merger will be governed by the Amended and Restated Articles of Incorporation and Second Amended and Restated Bylaws of First Interstate rather than by the articles of incorporation and bylaws of Mountain West. For more information, please see the section entitled “COMPARISON OF STOCKHOLDERS’ RIGHTS” beginning on page 73.

Dissenters’ Rights of Mountain West Stockholders (page 66)

As a stockholder of Mountain West, under Montana law you have the right to dissent from the merger and have the fair value of your shares of Mountain West common stock paid to you in cash. The fair value may be more or less than the value of the shares of First Interstate Class A Common Stock and cash being paid in the merger.

Persons having beneficial interests in Mountain West common stock held of record in the name of another person, such as a broker or bank, must act promptly to cause the record holder to take the actions required under Montana law to exercise your dissenters’ rights.

In order to dissent, you must carefully follow the requirements of the Montana Code Annotated, including giving the required written notice prior to the special meeting at which the vote on the merger agreement is taken. These steps are summarized under the caption “DISSENTERS’ RIGHTS OF MOUNTAIN WEST STOCKHOLDERS” on page 66.

If you intend to exercise dissenters’ rights, you should read the statutes carefully and consult with your own legal counsel. You should also remember that if you return a signed proxy card but fail to provide instructions as to how your shares of Mountain West common stock are to be voted, you will be considered to have voted in favor of the merger agreement and you will not be able to assert dissenters’ rights. Also, if you exercise dissenters’ rights, you may have taxable income as a result, so you are urged to consult with your own tax advisor if you intend to dissent. See the section entitled “MATERIAL UNITED STATES FEDERAL INCOME TAX CONSEQUENCES OF THE MERGER” on page 62. If the merger agreement is approved by the stockholders of Mountain West, holders of Mountain West common stock who make a written objection to the merger prior to the Mountain West special meeting, vote against the approval of the merger agreement, properly make a written demand for payment following notice of the merger and timely surrender their Mountain West stock certificates will be entitled to receive the fair value of their shares in cash under the Montana Code Annotated.

The text of the provisions of the Montana Code Annotated pertaining to dissenters’ rights is attached to this proxy statement/prospectus as Appendix C.

Risk Factors (page 19)

Before voting at the Mountain West special meeting, you should carefully consider all of the information contained in or incorporated by reference into this proxy statement/prospectus, including the risk factors set forth in the section entitled “RISK FACTORS” beginning on page 19 or described in First Interstate’s Annual Report on Form 10-K as of and for the year ended December 31, 2013 and other reports filed with the SEC, which are incorporated by reference into this proxy statement/prospectus. Please see the section entitled “WHERE YOU CAN FIND MORE INFORMATION” beginning on page ii.

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SELECTED HISTORICAL CONSOLIDATED FINANCIAL DATA FOR FIRST INTERSTATE

The following table summarizes consolidated financial results achieved by First Interstate for the periods and at the dates indicated and should be read in conjunction with First Interstate’s consolidated financial statements and the notes to the consolidated financial statements contained in reports that First Interstate has previously filed with the SEC. Historical financial information for First Interstate can be found in its Annual Report on Form 10-K for the year ended December 31, 2013. Please see the section entitled “WHERE YOU CAN FIND MORE INFORMATION” beginning on page ii for instructions on how to obtain the information that has been incorporated by reference. You should not assume the results of operations for the past periods presented indicate results for any future period.

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(1) For purposes of computing book value per share, book value equals common stockholders’ equity.

(2) Net interest margin ratio is presented on a fully taxable equivalent basis.

(3) Efficiency ratio represents non-interest expenses, excluding loan loss provision, divided by the aggregate of net interest income and non-interest income.

(4) Common stock dividend payout ratio represents dividends per share divided by basic earnings per share.

(5) Non-performing loans include nonaccrual loans (including nonaccrual troubled debt restructurings) and loans past due 90 days or more and still accruing interest.

(6) Non-performing assets include nonaccrual loans (including nonaccrual troubled debt restructurings), loans past due 90 days or more, and OREO.

(7) For purposes of computing tier 1 common capital to total risk weighted assets, tier 1 common capital is calculated as tier 1 capital less preferred stock and trust preferred securities.

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COMPARATIVE PER SHARE MARKET PRICE AND DIVIDEND INFORMATION

The tables below set forth, for the calendar quarters indicated, the high and low sales prices per share for, and the dividend paid per share with respect to, First Interstate Class A Common Stock, which is listed for trading on the NASDAQ Global Select Market under the symbol “FIBK,” and Mountain West common stock, for which no established public trading market exists. Though sporadic trading occurs in an over-the-counter market for Mountain West common stock, no registered broker/dealer makes a market in Mountain West common stock, and its over-the-counter market quotations reflect inter-dealer prices, without retail mark-up, mark-down or commission and may not necessarily represent actual transactions. The following table setting forth the ranges of trading prices for Mountain West’s common stock for the periods presented is provided to the extent known or believed to be known to the management of Mountain West.

* This dividend was declared by the Mountain West board of directors on December 30, 2013 and is to be paid on April 2, 2014 to stockholders of record as of February 28, 2014.

The following table sets forth the closing sale prices per share of First Interstate Class A Common Stock and Mountain West common stock on February 10, 2014, the last trading day before the public announcement of the signing of the merger agreement, and on March 31, 2014, the latest practicable date before the date of this document.

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CAUTIONARY STATEMENT REGARDING FORWARD-LOOKING STATEMENTS

Some of the statements in this document and the documents incorporated by reference herein contain forward-looking statements within the meaning of Section 27A of the Securities Act of 1933, as amended, or Securities Act, and Rule 175 promulgated thereunder, and Section 21E of the Securities Exchange Act of 1934, as amended, or Exchange Act, and Rule 3b-6 promulgated thereunder, that involve inherent risks and uncertainties.  These statements include statements about First Interstate’s and Mountain West’s plans, strategies and prospects and involve known and unknown risks that are difficult to predict. Therefore, actual results, performance or achievements may differ materially from those expressed in or implied by these forward-looking statements. In some cases, you can identify forward-looking statements by the use of words such as “may,” “could,” “expect,” “intend,” “plan,” “seek,” “anticipate,” “believe,” “estimate,” “predict,” “potential,” “continue,” “likely,” “will,” “would” and variations of these terms and similar expressions, or the negative of these terms or similar expressions.  You should carefully read forward-looking statements, including statements that contain these words, because they discuss the future expectations or state other “forward-looking” information about First Interstate, Mountain West and the combined company. Forward-looking statements speak only as of the date they are made. First Interstate and Mountain West assume no duty to update such statements.

Factors that may cause actual results to differ materially from current expectations are described in First Interstate’s reports filed with the SEC and the section of this document entitled “RISK FACTORS” (beginning on page 19), and include, but are not limited to:

·                  the ability to obtain regulatory approvals and meet other closing conditions to the merger, including approval by Mountain West stockholders, on the expected terms and schedule;

·                  a delay in closing the merger;

·                  difficulties and delays in integrating the First Interstate and Mountain West businesses or fully realizing cost savings and other benefits, including as a result of (i) potential negative publicity regarding consolidations or reductions in force, (ii) an inability to retain Mountain West employees, or (iii) attrition of Mountain West Bank customers;

·                  business disruption following the merger;

·                  diversion of management’s attention from ongoing business operations and opportunities;

·                  worsening economic conditions;

·                  adverse economic conditions affecting Montana, Wyoming and western South Dakota;

·                  credit losses;

·                  concentrations of real estate loans;

·                  commercial loan risk;

·                  adequacy of the allowance for loan losses;

·                  impairment of goodwill;

·                  changes in interest rates;

·                  access to low-cost funding sources;

·                  increases in deposit insurance premiums;

·                  inability to grow First Interstate’s business;

·                  the impact, extent and timing of technological changes, capital management activities, and other actions of the

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Federal Reserve, the Department of Business Oversight and the FDIC, and legislative and regulatory actions and reforms;

·                  governmental regulation and changes in regulatory, tax and accounting rule and interpretations;

·                  sweeping changes in regulation of financial institutions due to passage of Dodd-Frank Act;

·                  changes in or noncompliance with governmental regulations;

·                  effects of recent legislative and regulatory efforts to stabilize financial markets;

·                  dependence on management teams;

·                  ability to attract and retain qualified employees;

·                  failure of technology;

·                  reliance on external vendors;

·                  inability to meet liquidity requirements;

·                  lack of acquisition candidates;

·                  failure to manage growth;

·                  competition;

·                  inability to manage risks in turbulent and dynamic market conditions;

·                  ineffective internal operational controls;

·                  environmental remediation and other costs;

·                  litigation pertaining to fiduciary responsibilities;

·                  failure to effectively implement technology-driven products and services;

·                  capital required to support the bank subsidiaries, First Interstate Bank and Mountain West Bank;

·                  soundness of other financial institutions;

·                  impact of proposed Basel III capital standards for U.S. banks;

·                  inability of the bank subsidiaries, First Interstate Bank and Mountain West Bank, to pay dividends;

·                  implementation of new lines of business or new product or service offerings;

·                  change in dividend policies;

·                  volatility of First Interstate Class A Common Stock;

·                  voting control of First Interstate Class B stockholders;

·                  decline in market price of First Interstate Class A Common Stock;

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·                  dilution as a result of future equity issuances;

·                  uninsured nature of any investment in First Interstate Class A Common Stock and Mountain West common stock;

·                  the outcome of any legal proceedings that may be instituted against First Interstate or Mountain West;

·                  anti-takeover provisions;

·                  First Interstate controlled company status; and

·                  subordination of First Interstate Class A Common Stock to company debt.

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RISK FACTORS

In addition to the other information contained in or incorporated by reference into this proxy statement/prospectus, including the matters addressed under the caption entitled “CAUTIONARY STATEMENT REGARDING FORWARD-LOOKING STATEMENTS,” Mountain West stockholders should carefully consider the risk factors that appear in First Interstate’s (i) Annual Report on Form 10-K as of and for the year ended December 31, 2013 and other reports filed by First Interstate with the SEC, ,which are incorporated by reference into this proxy statement/prospectus and (ii) the following risk factors in deciding whether to vote for approval of the Mountain West Merger proposal. Stockholders of Mountain West should also consider the other information in this document and the other documents incorporated by reference into this document. Please see the sections entitled “WHERE YOU CAN FIND MORE INFORMATION” beginning on page ii and “INCORPORATION OF CERTAIN DOCUMENTS BY REFERENCE” beginning on page 79.

Because the market price of the First Interstate Class A Common Stock will fluctuate, you cannot be sure of the value of the merger consideration that you will receive.

First Interstate Class A Common Stock trades on the NASDAQ Global Market and has a market price that fluctuates.  As a result, you will not be able to precisely determine the value of the merger consideration you will receive upon completion of the merger at the time of the Mountain West special meeting of stockholders. Any change in the market price of First Interstate Class A Common Stock prior to completion of the merger will affect the value of the merger consideration that Mountain West stockholders will receive upon completion of the merger. First Interstate Class A Common Stock price changes may result from a variety of factors, including but not limited to general market and economic conditions, changes in First Interstate’s business, operations and prospects, and regulatory considerations. Many of these factors are beyond the control of First Interstate or Mountain West.

Mountain West stockholders may receive a form of consideration different from what they elect.

Although each Mountain West stockholder may elect to receive all cash or all First Interstate Class A Common Stock in the merger (rather than receive the established merger consideration consisting of a mix of cash and stock, for which no election need be made), there is a target aggregate number of shares of First Interstate Class A Common Stock that First Interstate is required to issue in the merger.  As a result, if either (i) the aggregate cash elections would result in the number of shares of First Interstate Class A Common Stock to be issued to be less than the target, and you chose the all cash election or (ii) the aggregate stock elections would result in the number of shares of First Interstate Class A Common Stock to be issued to exceed the target and you chose the all stock election, some or all of your consideration may be in a form that you did not choose.

The merger agreement limits Mountain West’s ability to pursue other transactions and provides for the payment of a break-up fee if Mountain West does so.

While the merger agreement is in effect and subject to very narrow exceptions, Mountain West and its directors, officers and agents are prohibited from initiating or encouraging inquiries with respect to alternative acquisition proposals. The prohibition limits Mountain West’s ability to seek offers that may be superior from a financial point of view from other possible acquirers. If Mountain West receives an unsolicited proposal from a third party that is superior from a financial point of view to that made by First Interstate and the merger agreement is terminated, Mountain West may be required to pay a $3 million break-up fee. This fee makes it less likely that a third party will make an alternative acquisition proposal.

The merger is subject to the receipt of consents and approvals from governmental entities that may impose conditions that could have an adverse effect on the combined company following the merger.

Before the merger may be completed, various approvals, consents and waivers must be obtained from the Federal Reserve Board and the Montana Division of Banking and Financial Institutions, and before the bank merger may be completed, various approvals, consents and waivers must additionally be obtained from the Federal Deposit Insurance Corporation and the Department of Justice. These governmental entities may impose conditions on the granting of such approvals and consents. Although First Interstate and Mountain West do not currently expect that any such material conditions or changes would be imposed, there can be no assurance that they will not be, and such conditions or changes could have the effect of delaying completion of the merger or imposing additional costs or limiting the revenues of the combined company following the merger, any of which might have an adverse effect on the combined company following the merger.

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First Interstate may fail to realize the anticipated benefits of the merger.

The success of the merger will depend on, among other things, First Interstate’s ability to combine the businesses of First Interstate and Mountain West. If First Interstate is not able to successfully achieve this objective, the anticipated benefits of the merger may not be realized fully, or at all, or may take longer to realize than expected.

First Interstate and Mountain West have operated and, until the consummation of the merger, will continue to operate independently. It is possible that the integration process or other factors could result in the loss or departure of key employees, the disruption of the ongoing business of First Interstate or inconsistencies in standards, controls, procedures and policies. It is also possible that clients, customers, depositors and counterparties of First Interstate could choose to discontinue their relationships with the combined company post-merger because they prefer doing business with an independent company or for any other reason, which would adversely affect the future performance of the combined company. These transition matters could have an adverse effect on each of First Interstate and Mountain West during the pre-merger period and for an undetermined time after the consummation of the merger.

The fairness opinion that Mountain West has obtained from Davidson, has not been, and is not expected to be, updated to reflect any changes in circumstances that will occur before the closing of the merger.

The fairness opinion issued on April 2, 2014 to Mountain West by Davidson, which is Mountain West’s financial advisor, regarding the fairness, from a financial point of view, of the consideration to be paid in connection with the merger, speaks only as of the date of this prospectus/proxy statement. Changes in the operations and prospects of Mountain West, general market and economic conditions and other factors which may be beyond the control of Mountain West, and on which the fairness opinion was based, may alter the value of Mountain West or the market price of shares of First Interstate as of the closing date of the merger. Davidson does not have any obligation to update, revise or reaffirm its opinion to reflect subsequent developments. Because Mountain West does not anticipate asking its financial advisor to update its opinion, the opinion will not address the fairness of the merger consideration from a financial point of view at the time the merger is completed. Mountain West’s board of directors’ recommendation that Mountain West stockholders vote “FOR” approval of the merger agreement, however, is made as of the date of this document. For a description of the opinion that Mountain West received from its financial advisor, see the section entitled “THE MERGER — Opinion of Mountain West’s Financial Advisor” included elsewhere in this proxy statement/prospectus.

Certain Mountain West directors and officers may have interests in the merger different from the interests of Mountain West stockholders.

In considering the recommendation of the board of directors of Mountain West, Mountain West stockholders should be aware that certain directors and executive officers of Mountain West have interests in the merger that may differ from, or be in addition to, the interests of Mountain West stockholders generally. These interests may arise from, among other things, salary continuation benefits and severance benefits that will be payable in the event of an employee’s termination within 24 months after the merger, an agreement that provides a payment upon a change in control and provisions in the merger agreement regarding continued indemnification and insurance for Mountain West and Mountain West Bank directors and officers. The board of directors of Mountain West was aware of these interests and considered them, among other things, when it approved the merger agreement and in making its recommendation that Mountain West stockholders approve the merger agreement and the merger.  For a more complete description of the interests of Mountain West directors and executive officers in the merger, see the section entitled “THE MERGER — Financial Interests of Directors and Officers of Mountain West in the Merger” included elsewhere in this proxy statement/prospectus.

Mountain West stockholders will have a reduced ownership and voting interest after the merger and will exercise less influence over management.

Mountain West stockholders currently have the right to vote in the election of the board of directors of Mountain West and on other matters affecting Mountain West. Upon the completion of the merger, each Mountain West stockholder who receives shares of First Interstate Class A Common Stock will become a stockholder of First Interstate with a percentage ownership of First Interstate that is smaller than such stockholder’s percentage ownership of Mountain West. The holders of First Interstate’s Class A Common Stock are entitled to one vote per share and the holders of First Interstate’s Class B Common Stock are entitled to five votes per share on any matter to be voted upon by the stockholders. Holders of First Interstate Class A Common Stock and Class B Common Stock vote together as a single class on all matters (including the election of directors) submitted to a vote of stockholders, unless otherwise required by law. It is currently expected that the former stockholders of

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Mountain West as a group will receive shares in the merger constituting approximately 3% of the combined shares of First Interstate Class A Common Stock and Class B Common Stock outstanding immediately after the merger. For these reasons, Mountain West stockholders will have less influence on the management and policies of First Interstate than they now have on the management and policies of Mountain West.

Mountain West will be subject to business uncertainties and contractual restrictions while the merger is pending.

Uncertainty about the effect of the merger on employees and customers may have an adverse effect on Mountain West and consequently on First Interstate. These uncertainties may impair Mountain West’s ability to attract, retain and motivate key personnel until the merger is completed, and could cause customers and others that deal with Mountain West to seek to change existing business relationships with Mountain West. Retention of certain employees may be challenging during the pendency of the merger, as certain employees may experience uncertainty about their future roles. If key employees depart because of issues relating to the uncertainty and difficulty of integration or a desire not to remain with the business, First Interstate’s business following the merger could be negatively impacted. In addition, the merger agreement restricts Mountain West from taking certain specified actions until the merger occurs without the consent of First Interstate. These restrictions may prevent Mountain West from pursuing attractive business opportunities that may arise prior to the completion of the merger. Please see the section entitled “THE MERGER AGREEMENT—Covenants and Agreements” beginning on page 49 for a description of the restrictive covenants applicable to Mountain West.

Regulatory approvals may not be received, may take longer than expected or may impose conditions that are not presently anticipated or cannot be met.

Before the merger and bank merger may be completed, various approvals must be obtained from the bank regulatory and other governmental authorities. These governmental entities may impose conditions on the granting of such approvals. Such conditions and the process of obtaining regulatory approvals could have the effect of delaying completion of the merger or of imposing additional costs or limitations on First Interstate following the merger. The regulatory approvals may not be received at any time, may not be received in a timely fashion, and may contain conditions on the completion of the merger that are not anticipated or cannot be met. Although First Interstate and Mountain West do not currently expect that any such material conditions or changes would be imposed, there can be no assurance that they will not be, and such conditions or changes could have the effect of delaying completion of the merger or imposing additional costs or limiting the revenues of the combined company following the merger, any of which might have an adverse effect on the combined company following the merger.

Branch consolidation could have an adverse effect on the combined company.

If the merger closes by or before the third quarter of 2014, First Interstate expects to consolidate six to eight branches, mostly at locations outside of the Helena market, by the end of 2014.  Such consolidations and, to the extent they occur, any associated reductions in force could result in negative publicity for the combined company, which could have an adverse effect on the combined company following the merger.

The combined company expects to incur substantial expenses related to the merger.

The combined company expects to incur substantial expenses in connection with consummation of the merger and combining the business, operations, networks, systems, technologies, policies and procedures of the two companies. Although First Interstate and Mountain West have assumed that a certain level of transaction and combination expenses would be incurred, there are a number of factors beyond their control that could affect the total amount or the timing of their combination expenses. Many of the expenses that will be incurred, by their nature, are difficult to estimate accurately at the present time. Due to these factors, the transaction and combination expenses associated with the merger could, particularly in the near term, exceed the savings that the combined company expects to achieve from the elimination of duplicative expenses, the realization of economies of scale and the cost savings related to the combination of the businesses following the consummation of the merger. As a result of these expenses, both First Interstate and Mountain West expect to take charges against their earnings before the completion of the merger and First Interstate expects to take additional charges after completion of the merger.

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MOUNTAIN WEST SPECIAL MEETING OF STOCKHOLDERS

Date, Time and Place

The special meeting of stockholders of Mountain West will be held at 10:00 a.m., Mountain Time, on May 13, 2014, at the Holiday Inn — Downtown, 22 N. Last Chance Gulch, Helena, MT 59601. On or about April 14, 2014, Mountain West commenced mailing this document and the enclosed form of proxy to its stockholders entitled to vote at the Mountain West special meeting.

Purpose of Mountain West Special Meeting

At the Mountain West special meeting, Mountain West stockholders will be asked to:

·                  approve the Mountain West Merger proposal; and

·                  approve the Mountain West Adjournment proposal, if necessary or appropriate.

Recommendation of the Mountain West Board of Directors

The Mountain West board of directors unanimously recommends that you vote “FOR” the Mountain West Merger proposal and “FOR” the Mountain West Adjournment proposal. Please see the section entitled “THE MERGER—Recommendation of the Mountain West Board of Directors and Reasons for the Merger” beginning on page 33.

Mountain West Record Date and Quorum

The Mountain West board of directors has fixed the close of business on March 31, 2014, as the record date for determining the holders of Mountain West common stock entitled to receive notice of and to vote at the Mountain West special meeting.

As of the Mountain West record date, there were 5,400,587 shares of Mountain West common stock outstanding and entitled to vote at the Mountain West special meeting held by approximately 330 holders of record. Each share of Mountain West common stock entitles the holder to one vote at the Mountain West special meeting on each proposal to be considered at the Mountain West special meeting.

The representation (in person or by proxy) of holders of at least a majority of the votes entitled to be cast on the matters to be voted on at the Mountain West special meeting constitutes a quorum for transacting business at the Mountain West special meeting. All shares of Mountain West common stock, whether present in person or represented by proxy, including abstentions and broker non-votes, will be treated as present for purposes of determining the presence or absence of a quorum for all matters voted on at the Mountain West special meeting.

Required Vote

Required Vote to Approve the Mountain West Merger Proposal

The affirmative vote of two-thirds of the outstanding shares of Mountain West common stock entitled to vote is required to approve the Mountain West Merger proposal.

Required Vote to Approve the Mountain West Adjournment Proposal

Assuming a quorum is present, the affirmative vote of a majority of the shares of Mountain West common stock represented (in person or by proxy) at the Mountain West special meeting and entitled to vote on the proposal is required to approve the Mountain West Adjournment proposal.

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Treatment of Abstentions; Failure to Vote

For purposes of the Mountain West special meeting, an abstention occurs when a Mountain West stockholder attends the Mountain West special meeting, either in person or by proxy, but abstains from voting.

·                  For the Mountain West Merger proposal, an abstention or a failure to vote will have the same effect as a vote cast “AGAINST” this proposal.

·                  For the Mountain West Adjournment proposal, assuming a quorum is present, if a Mountain West stockholder present in person at the Mountain West special meeting abstains from voting, or responds by proxy with an “abstain” vote, it will have the same effect as a vote cast “AGAINST” this proposal.

Voting on Proxies; Incomplete Proxies

Giving a proxy means that a Mountain West stockholder authorizes the persons named in the enclosed proxy card to vote its shares at the Mountain West special meeting in the manner it directs. A Mountain West stockholder may vote by proxy or in person at the Mountain West special meeting. If you hold your shares of Mountain West common stock in your name as a stockholder of record, to submit a proxy you, as a Mountain West stockholder, may use one of the following methods:

·                  Through the Internet: Use the Internet to vote your proxy 24 hours a day, 7 days a week. Have your proxy card handy when you access the website. You will be prompted to enter your control number(s), which is located on your proxy card, to create and submit an electronic ballot.

·                  By mail: Complete and return the proxy card in the enclosed envelope. The envelope requires no additional postage if mailed in the United States.

Mountain West requests that Mountain West stockholders vote over the Internet or by completing and signing the accompanying proxy and returning it to Mountain West as soon as possible in the enclosed postage-paid envelope. When the accompanying proxy is returned properly executed, the shares of Mountain West stock represented by it will be voted at the Mountain West special meeting in accordance with the instructions contained on the proxy card.

If any proxy is returned without indication as to how to vote, the shares of Mountain West common stock represented by the proxy will be voted as recommended by the Mountain West board of directors. Unless a Mountain West stockholder checks the box on its proxy card to withhold discretionary authority, the proxyholders may use their discretion to vote on any other matters voted upon at the Mountain West special meeting.

If a Mountain West stockholder’s shares are held in “street name” by a broker, bank or other nominee, the stockholder should check the voting form used by that firm to determine whether it may vote over the Internet.

Every Mountain West stockholder’s vote is important. Accordingly, each Mountain West stockholder should sign, date and return the enclosed proxy card, or vote via the Internet, whether or not the Mountain West stockholder plans to attend the Mountain West special meeting in person.

Shares Held in Street Name

If you are a Mountain West stockholder and your shares are held in “street name” through a bank, broker or other holder of record, you must provide the record holder of your shares with instructions on how to vote your shares. Please follow the voting instructions provided by the bank or broker. You may not vote shares held in street name by returning a proxy card directly to Mountain West or by voting in person at the Mountain West special meeting unless you provide a “legal proxy,” which you must obtain from your broker, bank or other nominee. Further, brokers, banks or other nominees who hold shares of Mountain West common stock on behalf of their customers may not give a proxy to Mountain West to vote those shares with respect to any of the proposals without specific instructions from their customers, as brokers, banks and other nominees do not have discretionary voting power on these matters. Therefore, if you are a Mountain West stockholder and you do not instruct your broker, bank or other nominee on how to vote your shares:

·                  your broker, bank or other nominee may not vote your shares on the Mountain West Merger proposal, which

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broker non-votes will have the same effect as a vote “AGAINST” this proposal; and

·                  your broker, bank or other nominee may not vote your shares on the Mountain West Adjournment proposal, which broker non-votes will have no effect on the vote count for this proposal.

Revocability of Proxies and Changes to a Mountain West Stockholder’s Vote

A Mountain West stockholder has the power to change his, her, or its vote at any time before his, her, or its shares of Mountain West common stock are voted at the Mountain West special meeting by:

·                  sending a notice of revocation to Mountain West Financial Corp., Attention: Corporate Secretary, P.O. Box 6013, Helena, MT 59604-6013, stating that you would like to revoke your proxy;

·                  logging onto the Internet website specified on your proxy card in the same manner you would to submit your proxy electronically if you are eligible to do so, and following the prompts;

·                  sending a completed proxy card bearing a later date than your original proxy card; or

·                  attending the Mountain West special meeting and voting in person.

If you choose any of the first three methods, you must take the described action no later than the beginning of the Mountain West special meeting. If you choose to send a completed proxy card bearing a later date than your original proxy card or a notice of revocation, the new proxy card or notice of revocation must be received before the beginning of the Mountain West special meeting. If you have instructed a bank, broker or other nominee to vote your shares of Mountain West common stock, you must follow the directions you receive from your bank, broker or other nominee in order to change or revoke your vote.

Solicitation of Proxies

The cost of solicitation of proxies from Mountain West stockholders will be borne by Mountain West. Mountain West will reimburse brokerage firms and other custodians, nominees and fiduciaries for reasonable expenses incurred by them in sending proxy materials to the beneficial owners of common stock. In addition to solicitations by mail, Mountain West’s directors, officers and regular employees may solicit proxies personally or by telephone without additional compensation.

Attending the Mountain West Special Meeting

Subject to space availability, all Mountain West stockholders as of the record date, or their duly appointed proxies, may attend the Mountain West special meeting.

If you hold your shares of Mountain West common stock in your name as a stockholder of record and you wish to attend the Mountain West special meeting, please bring valid picture identification.

If your shares of Mountain West common stock are held in “street name” in a stock brokerage account or by a bank or nominee and you wish to attend the Mountain West special meeting, you need to bring a copy of a bank or brokerage statement to the Mountain West special meeting reflecting your stock ownership as of the record date. You must also bring valid picture identification.

Voting and Support Agreements

Concurrently with and as a condition to First Interstate entering into the merger agreement, on February 10, 2014, all current directors and certain senior executive officers (and their affiliates) of Mountain West, holding approximately 53.6% of outstanding Mountain West common stock as of the record date, entered into Voting and Support Agreements to vote shares for approval of the merger and the merger agreement and against any competing acquisition proposals, and to refrain from selling such shares, subject to the terms of the Voting and Support Agreements.

As of the record date, First Interstate directors and executive officers beneficially held no shares of Mountain West common stock.

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MOUNTAIN WEST PROPOSALS

Mountain West Merger Proposal

As discussed throughout this proxy statement/prospectus, Mountain West is asking its stockholders to approve the Mountain West Merger proposal. Holders of Mountain West common stock should read carefully this proxy statement/prospectus in its entirety, including the appendices, for more detailed information concerning the merger agreement and the merger. In particular, holders of Mountain West common stock are directed to the merger agreement, a copy of which is attached as Appendix A to this proxy statement/prospectus.

Vote Required and Mountain West Board Recommendation

The affirmative vote of two-thirds of the outstanding shares of Mountain West common stock entitled to vote is required to approve the Mountain West Merger proposal.

The Mountain West board of directors unanimously recommends a vote “FOR” the Mountain West Merger proposal.

Mountain West Adjournment Proposal

The Mountain West special meeting may be adjourned to another time or place, if necessary or appropriate, to solicit additional proxies if there are insufficient votes at the time of the Mountain West special meeting to approve the Mountain West Merger proposal.

If, at the Mountain West special meeting, the number of shares of Mountain West common stock present or represented and voting in favor of the Mountain West Merger proposal is insufficient to approve the Mountain West Merger proposal, Mountain West intends to move to adjourn the Mountain West special meeting in order to enable the Mountain West board of directors to solicit additional proxies for approval of the merger. In that event, Mountain West will ask its stockholders to vote only upon the Mountain West Adjournment proposal, and not the Mountain West Merger proposal.

In the Mountain West Adjournment proposal, Mountain West is asking its stockholders to authorize the holder of any proxy solicited by the Mountain West board of directors to vote in favor of granting discretionary authority to the proxy holders, to adjourn the Mountain West special meeting to another time and place for the purpose of soliciting additional proxies. If the Mountain West stockholders approve the Mountain West Adjournment proposal, Mountain West could adjourn the Mountain West special meeting and use the additional time to solicit additional proxies, including the solicitation of proxies from Mountain West stockholders who have previously voted.

Vote Required and Mountain West Board Recommendation

Assuming a quorum is present, the affirmative vote of a majority of the shares of Mountain West common stock represented (in person or by proxy) at the Mountain West special meeting and entitled to vote on the proposal is required to approve the Mountain West Adjournment proposal.

The Mountain West board of directors recommends a vote “FOR” the Mountain West Adjournment proposal.

Other Matters to Come Before the Mountain West Special Meeting

No other matters are intended to be brought before the Mountain West special meeting by Mountain West, and Mountain West does not know of any matters to be brought before the Mountain West special meeting by others. If, however, any other matters properly come before the Mountain West special meeting, the persons named in the proxy will vote the shares represented thereby in accordance with their best judgment on any such matter.

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INFORMATION ABOUT THE COMPANIES

First Interstate BancSystem, Inc.
401 North 31st Street
Billings, Montana
Phone: (406) 255-5390

First Interstate is a financial and bank holding company headquartered in Billings, Montana and registered under the BHC Act. As of December 31, 2013, First Interstate had consolidated assets of $7.6 billion, deposits of $6.1 billion, loans of $4.3 billion and total stockholders’ equity of $802 million. As of the date of this document, First Interstate operates 74 banking offices, including detached drive-up facilities, in 41 communities located in Montana, Wyoming and western South Dakota. Through First Interstate Bank, First Interstate delivers a comprehensive range of banking products and services to individuals, businesses, municipalities and other entities throughout First Interstate’s market areas. First Interstate’s customers participate in a wide variety of industries, including energy, healthcare and professional services, education and governmental services, construction, mining, agriculture, retail and wholesale trade and tourism.

First Interstate’s stock is traded on the NASDAQ Global Select Market under the symbol “FIBK.”  Additional information about First Interstate and its subsidiaries may be found in the documents incorporated by reference into this proxy statement/prospectus. Please also see the section entitled “WHERE YOU CAN FIND MORE INFORMATION” beginning on page ii.

Mountain West Financial Corp.

1225 Cedar Street

Helena, Montana
Phone: (406) 449-2265

Mountain West is a bank holding company headquartered in Helena, Montana and registered under the BHC Act.  As of December 31, 2013, Mountain West had consolidated assets of $646 million, deposits of $540 million, loans of $405 million and total stockholders’ equity of $62 million. As of the date of this document, Mountain West operates 12 branches in five primary markets in Montana — Helena, Great Falls, Missoula, the Flathead Valley (Kalispell and Whitefish) and Bozeman. Through Mountain West Bank, Mountain West offers a full range of commercial and consumer banking products and services to individuals, businesses and organizations in Montana.

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THE MERGER

The following is a discussion of the merger and the material terms of the merger agreement between First Interstate and Mountain West. You are urged to read carefully the merger agreement in its entirety, a copy of which is attached as Appendix A to this proxy statement/prospectus and incorporated by reference herein. This summary does not purport to be complete and may not contain all of the information about the merger agreement that is important to you. This section is not intended to provide you with any factual information about First Interstate or Mountain West. Such information can be found elsewhere in this proxy statement/prospectus and in the public filings First Interstate makes with the SEC, as described in the section entitled “WHERE YOU CAN FIND MORE INFORMATION” beginning on page ii.

Terms of the Merger

Effect of the Merger

First Interstate’s and Mountain West’s boards of directors have approved the merger agreement. The merger agreement provides for the merger of Mountain West with and into First Interstate, with First Interstate continuing as the surviving corporation. If the merger is consummated, First Interstate and Mountain West expect that within six months Mountain West Bank will merge with and into First Interstate Bank, a bank chartered under the laws of the State of Montana and a wholly owned subsidiary of First Interstate. First Interstate Bank will be the surviving bank following the bank merger.

If the stockholders of Mountain West approve the merger agreement at the special meeting, and if the required regulatory approvals are obtained and the other conditions to the parties’ obligations to effect the merger are met or waived by the party entitled to do so, First Interstate and Mountain West anticipate that the merger will be completed in mid-2014, although delays could occur. As a result of the merger, holders of Mountain West common stock will be entitled to receive shares of First Interstate Class A Common Stock and cash, with cash paid in lieu of a fractional share, and will no longer be owners of Mountain West stock. In the merger, Mountain West stockholders will have the right, subject to the proration and allocation procedures described below, with respect to each of their shares of Mountain West common stock, to elect to receive all stock or all cash rather than the combination stock and cash merger consideration.  As a result of the merger, certificates for Mountain West common stock will only represent the right to receive the merger consideration pursuant to the merger agreement, and otherwise will be null and void after completion of the merger.

The value of the merger consideration that you will receive will fluctuate with the market price of First Interstate Class A Common Stock.

Merger Consideration

Under the merger agreement, at the effective time of the merger, each issued and outstanding share of Mountain West common stock will be cancelled and converted into the right to receive 0.2552 shares of First Interstate Class A Common Stock plus $7.125 in cash or, if a Mountain West stockholder properly elects and subject to the limitations summarized below, an amount in all cash or all stock roughly equal in value to the above described combination of stock and cash merger consideration. Based on the closing price of First Interstate Class A Common Stock on the NASDAQ Global Select Market on March 31, 2014 of $28.22 per share, and on the number of shares and options to purchase shares of Mountain West common stock issued and outstanding on such date, the aggregate value of the consideration that would be expected to be delivered to Mountain West stockholders in the merger would be approximately $78.8 million, of which $39.9 million would be paid in cash and $38.9 million would be represented by 1,378,230 shares of First Interstate Class A Common Stock.  The actual value received by Mountain West stockholders in the aggregate and on a per share of Mountain West common stock basis will fluctuate prior to the completion of the merger based on the prevailing market price of the First Interstate Class A Common Stock. You may obtain current stock price quotations for First Interstate Class A Common Stock on the NASDAQ Global Select Market under the symbol “FIBK.”  Mountain West common stock is not listed on a national securities exchange, and therefore current Mountain West stock price quotations are not available.

As an accommodation to Mountain West stockholders, First Interstate has agreed to allow them to elect to receive their individual merger consideration in either all cash or all First Interstate Class A Common Stock, subject to the limitations discussed below and in lieu of the stock and cash combination merger consideration.  Electing Mountain West stockholders may not ultimately receive in the merger only cash or only stock, however, because the merger agreement provides that First Interstate is only required to issue that number of shares of First Interstate Class A Common Stock that would have been issued in the merger if all Mountain West shares were converted at the specified stock and cash combination merger consideration

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(referred to herein as the target share number).  If, after giving effect to the merger consideration with respect to “non-electing” Mountain West stockholders, the elections received would result in the issuance of shares of First Interstate Class A Common Stock at the target share number, then the Mountain West shares shall be converted in accordance with the elections.  If, after giving effect to the merger consideration with respect to “non-electing” Mountain West stockholders, the elections received would result in the issuance of shares of First Interstate Class A Common Stock in excess of the target share number, then the number of shares of First Interstate Class A Common Stock to be delivered to the Mountain West stockholders making the all stock elections will be reduced proportionately and cash in an equivalent value will be paid to those stockholders.  If, after giving effect to the merger consideration with respect to “non-electing” Mountain West stockholders, the elections received would result in the issuance of fewer shares of First Interstate Class A Common Stock than the target share number, then the amount of cash to be delivered to the Mountain West stockholders making the all cash elections will be reduced proportionately and shares of First Interstate Class A Common Stock having an equivalent value will be issued to those stockholders.

No fractional shares of First Interstate Class A Common Stock will be issued in the merger.  A Mountain West stockholder will receive cash in lieu of any fractional shares of First Interstate Class A Common Stock such holder would otherwise have been entitled to receive in the merger. The amount of cash will be determined by multiplying the fractional share interest (after taking into account all shares of First Interstate Class A Common Stock held and rounded down to the nearest ten-thousandth when expressed in decimal form) by the average closing price of First Interstate’s Class A Common Stock on the NASDAQ Global Market during the twenty (20) business days preceding the fifth business day prior to the closing of the merger (referred to herein as the average closing price).

Stock Election

The merger agreement provides that each Mountain West stockholder who makes a valid election to receive 100% stock as consideration in the merger, which is referred to as a stock election, will have the right to receive, in exchange for each share of Mountain West common stock, subject to proration and adjustment as described below, an amount of shares of First Interstate Class A Common Stock equal to (i) 0.2552 plus (ii) the quotient of (x) $7.125 divided by (y) the average closing price. As discussed above, no fractional shares of First Interstate Class A Common Stock will be issued in the merger, and a holder of Mountain West common stock who would otherwise be entitled to a fractional share of First Interstate Class A Common Stock will receive cash in lieu thereof.

Cash Election

The merger agreement provides that each Mountain West stockholder who makes a valid election to receive 100% cash as consideration in the merger, which is referred to as a cash election, will have the right to receive, in exchange for each share of Mountain West common stock, subject to proration and adjustment as described below, an amount equal to (i) $7.125 plus (ii) the product of (x) 0.2552 and (y) the average closing price.

No Election

Mountain West stockholders who would like to receive the specified merger consideration consisting of a stock and cash combination do not need to make an election. Mountain West stockholders who make no election to receive cash or shares of First Interstate Class A Common Stock in the merger, whose elections are not received by the exchange agent by the election deadline, or whose forms of election are improperly completed and/or are not signed will be deemed not to have made an election. Shares of Mountain West common stock with respect to which no election is deemed to have been made are referred to herein as no election shares and will have the right to receive, in exchange for each share of Mountain West common stock, 0.2552 shares of First Interstate Class A Common Stock plus $7.125 in cash.

The requirement to elect all cash, all stock, or to make no election and receive the designated mix of cash and stock does not apply to the Mountain West Financial Corp. Profit Sharing Plan and Trust, which may make an election in any proportion of cash and stock to the extent deemed necessary or appropriate by the trustees or other fiduciaries of such plan to comply with applicable law.

Proration Procedures

The merger agreement provides a target aggregate number of shares of First Interstate Class A Common Stock that may be issued in the merger, which target equals that number of shares of First Interstate Class A Common Stock that would have been issued in the merger if all Mountain West shares were converted at the specified stock and cash combination merger

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consideration (referred to herein as the target share number). If the elections of the Mountain West stockholders would result in the aggregate number of shares of First Interstate Class A Common Stock to be issued to be greater or less than the target share number, then the Mountain West stockholder elections will be subject to adjustment as described below. Furthermore, the market value of the First Interstate Class A Common Stock (measured with reference to the lesser of the closing price of First Interstate Class A Common Stock on (i) February 7, 2014 and (ii) the closing date) issued in the merger may not be less than forty percent of the total consideration paid by First Interstate in the merger.

The exchange agent shall give effect to the election statements within five business days after the closing of the merger in accordance with the election statements as follows:

No Adjustment if Stock Consideration is Sufficiently Subscribed

If the election statements would result in (i) the market value of the aggregate amount of First Interstate Class A Common Stock (measured with reference to the lesser of the closing price of First Interstate Class A Common Stock on (a) February 7, 2014 and (b) the closing date) issued in the merger to be more than forty percent of the total consideration paid by First Interstate in the merger and (ii) the number of shares of First Interstate Class A Common Stock being equal to the target share number, then:

·                  a Mountain West stockholder making a cash election will receive 100% cash for each share of Mountain West common stock held by such holder in an amount equal to (i) $7.125 plus (ii) the product of (x) 0.2552 and (y) the average closing price;

·                  a Mountain West stockholder making a stock election will receive 100% stock for each share of Mountain West common stock held by such holder in an amount of shares of First Interstate Class A Common Stock equal to (i) 0.2552 plus (ii) the quotient of (x) $7.125 divided by (y) the average closing price; and

·                  all no election shares will be converted into the right to receive 0.2552 shares of First Interstate Class A Common Stock and $7.125 in cash per share.

Proration Adjustment if Stock Consideration is Oversubscribed

If the election statements would result in the number of shares of First Interstate Class A Common Stock that would be issued to be greater than the target share number, then:

·                  all no election shares will be converted into the right to receive 0.2552 shares of First Interstate Class A Common Stock and $7.125 in cash per share;

·                  a Mountain West stockholder making a cash election will receive 100% cash for each share of Mountain West common stock held by such holder in an amount equal to (i) $7.125 plus (ii) the product of (x) 0.2552 and (y) the average closing price;

·                  the exchange agent shall then adjust the election statements to reduce the number of shares of First Interstate Class A Common Stock to be delivered to Mountain West stockholders making all stock elections proportionally and shall pay cash in an equivalent value to those stockholders; and

·                  the exchange agent shall then, if necessary, adjust the election statements (and no election shares if necessary) by any reasonable method in order to cause the market value of the aggregate amount of First Interstate Class A Common Stock (measured with reference to the lesser of the closing price of First Interstate Class A Common Stock on (a) February 7, 2014 and (b) the closing date) to be issued to be as close as reasonably practical to, but in any event not less than, forty percent of the total consideration paid by First Interstate in the merger.

Proration Adjustment if Stock Consideration is Undersubscribed

If the election statements would result in the number of shares of First Interstate Class A Common Stock that would be issued to be less than the target share number, then:

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·      all no election shares will be converted into the right to receive 0.2552 shares of First Interstate Class A Common Stock and $7.125 in cash per share;

·      a Mountain West stockholder making a stock election will receive 100% stock for each share of Mountain West common stock held by such holder in an amount of shares of First Interstate Class A Common Stock equal to (i) 0.2552 plus (ii) the quotient of (x) $7.125 divided by (y) the average closing price; and

·      the exchange agent shall then adjust the election statements to reduce the amount of cash to be paid to Mountain West stockholders making all cash elections proportionally and shall deliver shares of First Interstate Class A Common Stock having an equivalent value to those stockholders; and

·      the exchange agent shall then, if necessary, adjust the election statements (and no election shares if necessary) by any reasonable method in order to cause the market value of the aggregate amount of First Interstate Class A Common Stock (measured with reference to the lesser of the closing price of First Interstate Class A Common Stock on (a) February 7, 2014 and (b) the closing date) to be issued to be as close as reasonably practical to, but in any event not less than, forty percent of the total consideration paid by First Interstate in the merger.

Conversion of Shares; Exchange of Certificates; Elections as to Form of Consideration

The conversion of Mountain West common stock into the right to receive the merger consideration will occur automatically at the effective time of the merger. As soon as reasonably practicable after the effective time of the merger, the exchange agent will exchange certificates representing shares of Mountain West common stock for merger consideration to be received in the merger pursuant to the terms of the merger agreement.

Election Statement

An election statement is being distributed with this proxy statement/prospectus to each holder of record of Mountain West common stock as of record date for the Mountain West meeting permitting each holder of Mountain West common stock the ability to make an all cash or all stock election with respect to the type of merger consideration they wish to receive.

Mountain West stockholders who want to receive the specified merger consideration consisting of a stock and cash combination do not need to make an election.

Holders of Mountain West common stock who wish to elect the type of merger consideration they will receive in the merger should carefully review and follow the instructions set forth in the election statement. Mountain West stockholders who hold their shares in “street name” should follow their broker’s instructions for making an election with respect to such shares. All election statements must be received by the exchange agent by 5:00 p.m., Mountain Time, on the date five days prior to the completion of the merger. This date is referred to as the election deadline. Shares of Mountain West common stock as to which the holder has not made a valid election prior to the election deadline will be treated as no election shares.

The actual election deadline is not currently known. First Interstate and Mountain West will issue a press release announcing the date of the election deadline at least five business days prior to that deadline. Additionally, First Interstate and Mountain West will post the date of the election deadline on their respective web sites, also at least five business days before that deadline.

To make an election, a holder of Mountain West common stock must submit a properly completed election statement so that it is actually received by the exchange agent at or prior to the election deadline in accordance with the instructions on the election statement. Neither Mountain West nor First Interstate is under any obligation to notify any holder of defects in such holder’s election statement.

Generally, an election may be revoked or changed, but only by written notice received by the exchange agent prior to the election deadline. If an election is revoked and unless a subsequent properly executed election statement is actually received by the exchange agent at or prior to the election deadline, the holder having revoked the election will be deemed to have made no election with respect to his or her shares of Mountain West common stock.

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Holders will not be entitled to revoke or change their elections following the election deadline. As a result, holders who have made elections will be unable to revoke their elections.

Shares of Mountain West common stock as to which the holder has not made a valid election prior to the election deadline, including as a result of revocation, will be deemed to have made no election. If it is determined that any purported cash election or stock election was not properly made, the purported election will be deemed to be of no force or effect and the holder making the purported election will be deemed not to have made an election for these purposes, unless a proper election is subsequently made on a timely basis.

Anti-Dilution

If prior to the effective time of the merger, First Interstate or Mountain West changes the number of outstanding shares of its capital stock by way of a reclassification, recapitalization, stock split, subdivision, stock dividend, stock distribution or any other similar combination or readjustment of shares, the merger consideration, including the per share cash consideration, the per share stock consideration and all other dependent components of the merger consideration, will be adjusted proportionally to reflect such change.

Treatment of Mountain West Stock Options

Under the merger agreement, Mountain West has agreed that every option to purchase Mountain West common stock outstanding and not expired immediately prior to the completion of the merger, whether or not then vested and whether or not then exercisable, shall be cancelled in exchange for a payment in cash equal to (x) the per share merger consideration value, minus (y) the applicable exercise price per share, multiplied by (z) the number of shares available under such option, assuming 100% vesting.

Background of the Merger

The management and directors of Mountain West have from time to time considered and discussed financial and strategic options potentially available to Mountain West.  These discussions have focused on, among other things, the business, market and regulatory environment faced by community banks, competitive and operational challenges faced by Mountain West, ongoing consolidation in the financial services industry, and options to enhance stockholder value and liquidity. With continued improvement in Mountain West’s asset quality in the first half of 2013, and having received general unsolicited expressions of potential interest from two larger banking organizations, the directors of Mountain West determined to interview selected investment banking firms to explore the prospect of pursuing a sale or merger of Mountain West later in the year.

On May 23, 2013, representatives from the investment banking group of Davidson met with Mountain West’s Executive Committee of the board of directors to review information regarding the community bank operating environment generally, Mountain West’s financial condition and operating results relative to peers, bank valuation levels in the public market and in merger and acquisition transactions, the process required to pursue a sale transaction, potential acquirers or merger partners, and the prospective valuation of Mountain West under certain assumptions.  Following that presentation and further discussion among management, directors, and Fredrikson & Byron, P.A., outside counsel, the board determined to interview selected investment banks to represent Mountain West in a potential sale or merger transaction.

On June 11, June 24 and June 25, the Mountain West board received presentations from four investment banking firms, including Davidson.  Matters discussed included the firms’ backgrounds, capabilities and experience in bank sale transactions in the region, operating and market conditions affecting community banks, valuation levels for community banks generally and for Mountain West specifically, the timing and process required to pursue a merger or sale transaction, and the fee structures proposed by the investment banks.  Following discussion and consideration of the presentations, the board elected to hire Davidson as its investment banking advisor and on July 12, Mountain West and Davidson entered into an engagement letter agreement.

From July 13 to October 30, management and Davidson worked together to compile and analyze information regarding Mountain West, organize an electronic due diligence data room, prepare financial analyses, draft a confidential information memorandum describing Mountain West (including its financial condition, results and prospects), and identify potential acquirers or merger partners.  On October 22, Davidson provided a presentation to the Mountain West board of directors that included prevailing market conditions, updated guidance on a likely valuation range for Mountain West, a process overview and timeframe, and the parties that management and Davidson recommended for inclusion in the confidential

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marketing process.  After discussion, the board authorized management and Davidson to proceed with the process.

On October 31 and November 1, Davidson contacted the 15 financial institutions approved by the management and board of Mountain West to determine their interest in exploring a possible acquisition of a bank having the general characteristics of Mountain West.  Seven of the parties expressed potential interest and signed confidentiality agreements to obtain additional information regarding Mountain West on a confidential basis.  From November 4 until late November, Davidson participated in discussions with these parties and answered questions or provided supplemental information.  Davidson requested non-binding indications of interest, including initial valuation ranges, by December 3.  On December 3, Davidson received three written proposals and two informal verbal indications of interest at a general valuation level.  The other two parties that had received the confidential information memorandum determined to not submit an indication of interest for reasons that included the size of Mountain West, the specific market areas served, and a focus on other strategic expansion opportunities.

On December 4, Davidson met with the board of directors of Mountain West to review the indications of interest and select the parties with which to move forward for further evaluation and discussions.  After discussion, the board authorized moving forward with the three parties that had submitted written proposals.  Davidson contacted these parties and provided them and their advisors with access to an extensive electronic due diligence data room containing information, documents and reports regarding Mountain West.  From December 5 to January 3, management of Mountain West and Davidson responded to information requests, answered questions, and added additional documents and information to the electronic data room.  During this time, management and Davidson also participated in teleconference meetings with the parties other than First Interstate to present and discuss information concerning Mountain West and to answer questions concerning asset quality, financial condition, and operating results and prospects.

On December 17, management of Mountain West and Davidson met with First Interstate in Billings, Montana. Participants in the meeting for Mountain West were Rick Hart, CEO, Dick Morgan, CFO, Mitch Bradshaw, Chief Credit Officer, and Loren Brown, Senior Vice President/Cashier.  Participants in the meeting for First Interstate were Tom Scott, Chairman, Jim Scott, Executive Vice Chairman, Ed Garding, CEO, Kevin Riley, CFO, and Bob Cerkovnik, Chief Credit Officer.  Participants also included representatives of the parties’ investment banking advisors.  The parties reviewed and discussed information concerning both Mountain West and First Interstate, including markets served, bank market conditions, operations, asset quality, and their respective financial condition, results and prospects.  The parties also discussed their community banking philosophies, commitments to communities, customers and employees, and the prospective benefits of a combination.

On December 20, Davidson sent the parties detailed instructions on the matters to be covered in their Letters of Intent to acquire Mountain West.  From December 20, 2013 through January 3, 2014, Davidson provided the parties with updated information concerning Mountain West’s financial condition, asset quality and estimated earnings, and discussed the prospect for improving the terms to their initial proposals.

On January 6, Davidson and Mountain West received detailed proposals from First Interstate and one other party. The third party elected to not submit a proposal after re-considering whether to expand into Mountain West’s market area versus focusing on organic growth and other opportunities in and around its existing banking markets. On January 7 and January 8, Davidson and Mountain West management reviewed and discussed the two submitted proposals with outside counsel and members of Mountain West’s Executive Committee of the board of directors. Discussions included the relative merits of the proposals and potential changes to the terms of the proposal from First Interstate.

On January 8, Mountain West’s board of directors and Davidson met to review and consider the proposals.  Discussions included bank market and valuation conditions generally, the process that was undertaken by Mountain West and Davidson over the preceding months to solicit competitive proposals from multiple parties, and the relative merits of the two detailed proposals received by Mountain West.  After discussion, the board determined that the proposal received from First Interstate was more favorable to Mountain West’s stockholders than the other proposal, and that Mountain West should enter into the Letter of Intent with First Interstate subject to the negotiation and acceptance of certain revisions to the terms of the January 6 proposal.  The revisions principally included clarification that the value to be received by Mountain West’s option holders would not affect the value received by stockholders, revised collar and walk-away provisions, reduced breakup fees, and revised terms for non-compete agreements and the provision of ongoing director and officer indemnification.

On January 9, Davidson supplied First Interstate with the proposed revisions and the parties further discussed and negotiated the changes to the Letter of Intent.  On January 10, First Interstate provided Mountain West a revised Letter of Intent reflecting the revised terms, and the parties entered into the Letter of Intent.

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From January 13 through January 23, the parties and their advisors exchanged additional information and engaged in continued discussions. From January 17 through January 20, the parties met in Helena to enable First Interstate to complete an extensive review of Mountain West’s loan files.  On January 17, Mr. Hart, Mr. Morgan and Mr. Riley also met to discuss operating and integration considerations and to tour Mountain West’s Helena banking offices.

From January 23 through February 7, the parties and their advisors exchanged and negotiated the terms of documents and schedules relating to the merger, including the Merger Agreement.

On February 10, the board of directors of Mountain West met in Helena to discuss and consider the proposed merger.  They were joined by representatives of Davidson and Fredrikson & Byron.  Matters discussed included the fiduciary responsibilities of the board, the background of the merger, the process undertaken to solicit merger or acquisition proposals, the provisions of the merger agreement and other documents related to the merger, business and financial information concerning Mountain West and First Interstate, valuation and financial analyses, and the matters described under “Recommendation of the Mountain West Board of Directors and Reasons for the Merger”.  Following discussion and the delivery by Davidson of the fairness opinion, the board voted unanimously in support of the merger with First Interstate.

Recommendation of the Mountain West Board of Directors and Reasons For The Merger

Mountain West’s board of directors has unanimously approved the merger agreement and unanimously recommends that the Mountain West stockholders vote “FOR” approval of the merger agreement.

At a special meeting held on February 10, 2014, the Mountain West board of directors unanimously determined that the terms of the merger agreement were fair to, and in the best interests of, Mountain West and its stockholders.  In the course of reaching its decision to approve the merger agreement, the board of directors consulted with Davidson, its financial advisor, and Fredrikson & Byron, P.A., its legal counsel.  In reaching its determination, the Mountain West board of directors considered a number of factors which constituted the reasons that the board of directors determined to approve the merger and to recommend that Mountain West stockholders vote in favor of the merger.  Such factors included the following:

·                  the process undertaken to arrive at the merger;

·                  the terms of the merger agreement and the value, form and mix of consideration to be received by Mountain West stockholders in the merger;

·                  the provisions of the merger agreement allowing Mountain West to pay a $0.33 per share cash dividend to its stockholders prior to consummation of the merger and a $0.06 per share cash dividend for each whole or partial calendar month elapsing prior to the effective time of the merger, beginning with September 2014;

·                  the provisions of the merger agreement allowing Mountain West to cause all outstanding stock options to be exercised or cashed out without affecting the value and consideration to be received by stockholders;

·                  the value to be received by Mountain West stockholders in the merger relative to the likely value of Mountain West on a stand-alone basis, the value of comparable companies, the financial terms of comparable transactions in the banking industry, the premium to be received over the quoted price for Mountain West shares on the OTC Bulletin Board, and the relative financial contributions of Mountain West and First Interstate;

·                  the opportunity for stockholders of Mountain West to achieve much greater liquidity for their shares of Mountain West stock through the payment of cash and delivery of shares of First Interstate Class A Common Stock that are actively traded on the NASDAQ Global Market;

·                  the historic and prospective business and financial results of First Interstate and the historic trading prices of First Interstate Class A Common Stock;

·                  the fact that Mountain West and First Interstate have a shared operating footprint and  similar business cultures, operating principles, and commitments to Montana communities;

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·                  the opportunity for Mountain West stockholders receiving shares of First Interstate Class A Common Stock to participate in the prospective growth and financial performance of the combined company, including the contemplated operational efficiencies following the merger;

·                  the fact that First Interstate has consistently paid dividends for approximately 20 years;

·                  the likelihood that First Interstate will be able to obtain regulatory approval of the merger;

·                  the historic and prospective business and financial results and future resource requirements of Mountain West;

·                  the current and prospective economic, operational, competitive and regulatory environment facing Mountain West and the financial services industry generally, including continued consolidation in the industry, increased regulatory and technology requirements, and the increased importance of operational scale and financial resources in maintaining efficiency and remaining competitive over the long-term;

·                  the opportunity for Mountain West customers to continue to receive personalized service and local decision-making, while benefiting from expanded locations, products, services, and lending capacities;

·                  the additional advancement and training opportunities for Mountain West employees in the combined company, and the commitments by First Interstate to mitigate the impact of employee reductions by (a) not hiring for open First Interstate positions in Mountain West markets between the date of the merger agreement and the closing of the merger (to create opportunities for Mountain West employees who might otherwise be displaced), and (b) providing generous severance and outplacement service arrangements for employees whose jobs are eliminated following the merger;

·                  the opinion of Davidson that the merger consideration to be received by Mountain West stockholders in the merger is fair from a financial point of view; and

·                  the provisions in the merger agreement that provide for the ability of the Mountain West board of directors to respond to an unsolicited acquisition proposal that the board of directors determines in good faith is a superior proposal, and to terminate the merger agreement, subject to certain conditions, including the payment of the Breakup Fee, if Mountain West has entered into a letter of intent or other agreement with respect to a superior proposal.

The Mountain West board of directors also considered a number of uncertainties and risks in its deliberations concerning the transactions contemplated by the merger agreement, including the following:

·                  that a portion of the merger consideration will be paid through the issuance of a fixed number of shares of First Interstate Class A Common Stock and any decrease in the market price of First Interstate Class A Common Stock will result in a reduction in the aggregate merger consideration to be received by Mountain West stockholders at the time of completion of the merger (subject to the adjustment procedures described under “The Merger Agreement —Termination of the Merger Agreement”);

·                  that at the time of the stockholders’ meeting, Mountain West stockholders will not necessarily know or be able to calculate the actual value of the merger consideration which they would receive upon completion of the merger;

·                  the possible disruption to business that may result from the announcement of the merger and the resulting distraction of management’s attention from the day-to-day operations of Mountain West;

·                  the contemplated branch consolidations and probable reduction in the number of Mountain West employees as a result of the operating efficiencies enabled by the merger;

·                  the restrictions contained in the merger agreement on the operation of business by Mountain West during the period between signing of the merger agreement and completion of the merger, as well as the other covenants and agreements of Mountain West contained in the merger agreement;

·                  that First Interstate has a right to a $1,000,000 termination fee in certain circumstances; and

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·                  that the $3,000,000 Breakup Fee provided for in the merger agreement could have the effect of discouraging superior proposals for a business combination between Mountain West and third parties.

The foregoing discussion of the reasons that led the Mountain West board of directors to unanimously approve the merger and unanimously recommend that Mountain West stockholders vote in favor of the merger is not intended to be exhaustive, but is believed to include all of the material reasons for the board of directors’ decision.  In reaching its determination to approve and recommend the transaction, the Mountain West board based its recommendation on the totality of the information presented to it and did not assign any relative or specific weights to the reasons considered in reaching that determination.  Individual directors may have given differing weights to different reasons.  After deliberating with respect to the merger with First Interstate, considering, among other things, the matters discussed above and the opinion of Davidson referred to above, the Mountain West board of directors unanimously approved and adopted the merger agreement and the merger with First Interstate.

For the reasons set forth above, the Mountain West board of directors has unanimously approved the merger agreement and the transactions contemplated thereby and recommends that you vote “FOR” the Mountain West Merger proposal and “FOR” the Mountain West Adjournment proposal (if necessary or appropriate).

Opinion of Mountain West’s Financial Advisor

Davidson was retained to act as financial advisor to Mountain West Financial Corp. in connection with the merger and to render an opinion as to whether the merger consideration was fair to the holders of Mountain West common stock from a financial point of view. At a meeting of Mountain West board of directors held on February 10, 2014, Davidson rendered its opinion to the effect that, based upon and subject to the considerations set forth in the opinion and based upon such other matters as Davidson considered relevant, the merger consideration was fair, from a financial point of view, to the stockholders of Mountain West as of the date of the opinion. Davidson has reconfirmed its fairness opinion as of the date of this prospectus/proxy statement.

The full text of the written opinion of Davidson dated April 2, 2014, which sets forth the procedures followed, assumptions made, matters considered and limitations on the review undertaken in connection with the opinion, is attached as Appendix B to this document and is incorporated herein by reference. Mountain West’s stockholders should read the opinion in its entirety. Davidson provided its opinion for the information and assistance of Mountain West’s board of directors in connection with its consideration of the Merger. The Davidson opinion is not a recommendation as to how any holder of Mountain West’s common stock should vote with respect to the Merger.

Davidson agreed to deliver its fairness opinion to the board of directors of Mountain West in connection with the execution of the merger agreement and to update its opinion for inclusion in this proxy statement/prospectus.  In connection with the update of the fairness opinion, Davidson performed procedures, as necessary, to update certain of the analyses described below.  Davidson reviewed the assumptions on which the analyses described below were based and the factors considered in connection with the analyses.  Davidson did not perform any analyses in addition to those described below in updating the February 10, 2014 opinion.

In connection with rendering its opinion and performing its related financial analyses, Davidson reviewed, among other things:

·                  the merger agreement;

·                  certain financial statements, financial analyses (including core earnings), and other historical financial and business information about Mountain West and First Interstate made available to us from published sources and/or from the internal records of Mountain West and  First Interstate that we deemed relevant;

·                  certain publicly available analyst earnings estimates for First Interstate for the years ending December 31, 2014 and December 31, 2015 and estimated growth rates for the years thereafter;

·                  the current market environment generally and the banking environment in particular;

·                  the regulatory, competitive and operating environment and related considerations affecting community banks generally and Mountain West in particular;

·                  future business and financial prospects for Mountain West on a stand-alone basis, based on management guidance;

·                  the net present value of Mountain West with consideration of projected financial results through 2018 based on management guidance;

·                  the relative contributions of Mountain West and First Interstate to the combined company;

·                  the financial terms of certain other similar transactions in the financial institutions industry, to the extent publicly available;

·                  the market, trading and valuation characteristics of Mountain West’s common stock, including comparisons to comparable public banks and bank holding companies;

·                  the market, trading, dividend payment and valuation characteristics of First Interstate’s Class A Common Stock, including comparisons to comparable public bank holding companies;

·                  the pro forma financial impact of the merger, taking into consideration the amounts and timing of transaction costs, earnings estimates, potential cost savings, and other financial and accounting considerations in connection with the merger; and

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·                  such other information, financial studies, analyses and investigations and financial, economic and market criteria as Davidson considered relevant, including discussions with management and other representatives and advisors of Mountain West and  First Interstate concerning the business, financial condition, results of operations and prospects of Mountain West and First Interstate.

In rendering its opinion, Davidson assumed and relied upon the accuracy and completeness of all information supplied or otherwise made available to Davidson, discussed with or reviewed by or for Davidson, or publicly available, and Davidson has not assumed responsibility for independently verifying such information or undertaken an independent evaluation or appraisal of any of the assets or liabilities (contingent or otherwise) of Mountain West or First Interstate, nor has Davidson been furnished with any such evaluation or appraisal. In addition, Davidson has not assumed any obligation to conduct, nor has it conducted any physical inspection of the properties or facilities of Mountain West or First Interstate. Davidson has further relied on statements from the management of Mountain West and First Interstate that they are not aware of any facts or circumstances that would make any of such information inaccurate or misleading. Davidson did not make an independent evaluation of the adequacy of the allowance for loan losses of Mountain West or First Interstate nor has Davidson reviewed any individual credit files relating to Mountain West or First Interstate. Davidson has assumed that the respective allowances for loan losses for both Mountain West and First Interstate are adequate to cover such losses and will be adequate on a pro forma basis for the combined entity. Davidson has assumed that there has been no material change in Mountain West’s or First Interstate’s assets, financial condition, results of operations, business or prospects since the date of the most recent financial statements provided.

Davidson has assumed in all respects material to the analysis that Mountain West and First Interstate will remain as going concerns for all periods relevant to the analysis. It has also assumed in all respects material to the analysis that all of the representations and warranties contained in the merger agreement and all related agreements are true and correct, that each party to such agreements will perform all of the covenants required to be performed by such party under such agreements and that the conditions precedent in the merger agreement will not be waived.

Davidson has assumed that in the course of obtaining necessary regulatory or other consents or approvals (contractual or otherwise) for the transaction, no restrictions, including any divestiture requirements or amendment or modifications, will be imposed that will have a material adverse effect on the contemplated benefits of the transaction.

Davidson does not express any view as to, and its opinion does not address, the relative merits of the transaction as compared to any alternative business strategies that might exist for Mountain West or the effect of any other transaction in which Mountain West might engage. Additionally, Davidson is not expressing any opinion herein as to the prices at which the shares of Mountain West or First Interstate currently trade or may trade in the future. The opinion of Davidson is necessarily based upon information available to Davidson and economic, market, financial and other conditions as they exist and can be evaluated on the date of the opinion.

Set forth below is a summary of the material financial analyses performed by Davidson in connection with rendering its opinion. The summary of the analyses of Davidson set forth below is not a complete description of the analysis underlying its opinion, and the order in which these analyses are described below is not indicative of any relative weight or importance given to those analyses by Davidson. The following summaries of financial analyses include information presented in tabular format. You should read these tables together with the full text of the summary financial analyses, as the tables alone are not a complete description of the analyses.

Unless otherwise indicated, the following quantitative information, to the extent it is based on market data, is based on market data as of February 6, 2014, and is not necessarily indicative of market conditions after such date.

Summary of Proposal

Davidson reviewed the financial terms of the proposed transaction.  The terms and conditions of the merger are more fully described in the merger agreement.  Each outstanding share of Mountain West common stock will be exchanged for (i) cash consideration of $7.125 and (ii) 0.2552 shares of First Interstate class A common stock, which equates to $13.45 per share based on First Interstate’s closing stock price of $24.78 on February 6, 2014.  Based upon Mountain West’s common shares outstanding at December 31, 2013 of 5,236,748, the aggregate consideration to Mountain West common stockholders is cash of

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$37.3 million and 1,336,418 shares of First Interstate Class A Common Stock.  In addition, all options that are outstanding at closing will be cashed out at the difference between the merger value per share and the weighted average strike price.  Based upon Mountain West’s options outstanding of 424,000 at December 31, 2013 with a weighted average strike price of $8.21, option holders would receive an aggregate of approximately $2.2 million in cash.  In total, Mountain West stockholders and option holders would receive total value of $72.7 million.  Based upon financial information as of or for the twelve month period ended December 31, 2013, Davidson calculated the following transaction ratios:

(1) Core earnings per share were calculated by making adjustments to Mountain West’s stated earnings to eliminate items of extraordinary or non-recurring income and expense, as confirmed with Mountain West management.

Stock Trading History of Mountain West and First Interstate

Davidson reviewed the history of the reported trading prices and volume of Mountain West and First Interstate common stock and the relationship between the movements in the prices of Mountain West and First Interstate common stock to movements in certain stock indices, including the Standard & Poor’s 500 Index, the Keefe, Bruyette & Woods, Inc. Regional Bank Index and the SNL Bank Index, and relative to their respective peers that are discussed in the Comparable Company Analysis.

Mountain West One Year Stock Performance