Exhibit 99.1 FIRST INTERSTATE ACCELERATES STRATEGIC EXPANSION INTO GROWTH MARKETS OF THE PACIFIC NORTHWEST WITH ACQUISITIONS OF IDAHO INDEPENDENT BANK AND COMMUNITY 1ST BANK OCTOBER 11, 2018

Safe Harbor This presentation contains forward-looking statements within the meaning of the Private Securities Litigation Reform Act of 1995. These forward- looking statements include, but are not limited to, statements about (1) the benefits of the mergers between First Interstate BancSystem, Inc. (“First Interstate”) and Idaho Independent Bank (“IIBK”) and Community 1st Bank (“CMYF”), including anticipated future results, cost savings and accretion to reported earnings that may be realized from the mergers; (2) First Interstate, IIBK’s and CMYF’s plans, objectives, expectations and intentions and other statements contained in this presentation that are not historical facts; and (3) other statements identified by words such as “expects,” “anticipates,” “intends,” “plans,” “believes,” “seeks,” “estimates” or words of similar meaning. Forward-looking statements involve risks and uncertainties that may cause actual results to differ materially from those in such statements. The following factors, among others, could cause actual results to differ materially from the anticipated results expressed in the forward-looking statements: the businesses of First Interstate, IIBK and CMYF may not be combined successfully, or such combination may take longer than expected; the cost savings from the mergers may not be fully realized or may take longer than expected; operating costs, customer loss and business disruption following the mergers may be greater than expected; governmental approvals of the mergers may not be obtained, or adverse regulatory conditions may be imposed in connection with governmental approvals of the mergers or otherwise; the stockholders of IIBK and CMYF may fail to approve the merger; credit and interest rate risks associated with First Interstate’s, IIBK’s and CMYF ’s respective businesses; and difficulties associated with achieving expected future financial results. Additional factors that could cause actual results to differ materially from those expressed in the forward-looking statements are discussed in First Interstate’s reports (such as the Annual Report on Form 10-K, Quarterly Reports on Form 10-Q and Current Reports on Form 8-K) filed with the SEC and available at the SEC’s Internet website (www.sec.gov). All subsequent written and oral forward-looking statements concerning the proposed transactions or other matters attributable to First Interstate, IIBK or CMYF or any person acting on their behalf are expressly qualified in their entirety by the cautionary statements above. Except as required by law, First Interstate, IIBK and CMYF do not undertake any obligation to update any forward-looking statement to reflect circumstances or events that occur after the date the forward-looking statement is made. 2

Important Additional Information This presentation is being made in respect of the proposed mergers between First Interstate and IIBK and First Interstate and CMYF. This presentation does not constitute an offer to sell or the solicitation of an offer to buy any securities or a solicitation of any vote or approval. In connection with the proposed transaction, First Interstate intends to file registration statements on Form S-4 with the SEC, which will include a proxy statement for each IIBK and CMYF, and a prospectus of First Interstate, and First Interstate, IIBK and CMYF will file other documents regarding the proposed transactions with the SEC. Before making any voting or investment decision, investors and security holders of IIBK and CMYF are urged to carefully read the entire registration statement and proxy statement/prospectus, when they become available, as well as any amendments or supplements to these documents, because they will contain important information about the proposed transactions. The documents filed by First Interstate with the SEC may be obtained free of charge at the SEC’s website at www.sec.gov. In addition, the documents filed by First Interstate may be obtained free of charge at its website at www.fibk.com or by contacting First Interstate BancSystem, Inc., 401 North 31st Street, Billings, Montana 59116, Attention: Marcy D. Mutch, Executive VP & CFO, telephone (406) 255-5390. First Interstate, IIBK and CMYF and certain of their directors and executive officers may be deemed to be participants in the solicitation of proxies of IIBK’s and CMYF ’s shareholders in connection with the proposed transactions. Information about the directors and executive officers of First Interstate and their ownership of First Interstate common stock is set forth in the proxy statement for First Interstate’s 2018 Annual Meeting of Shareholders, as filed with the SEC on Schedule 14A on March 16, 2018. Additional information regarding the interests of those participants and other persons who may be deemed participants in the transaction may be obtained by reading the proxy statement/prospectus regarding the proposed mergers when it becomes available. Free copies of this document may be obtained as described in the preceding paragraph. 3

Transaction Highlights o Continued geographic diversification into attractive, high growth markets in Pacific Northwest o Strong Idaho presence in Boise and Coeur d’Alene Strategically o Cohesive set of banking franchises with similar cultures due to the management, market and ownership Compelling o Strong deposit franchises with $725 million of 0.13% cost of deposits and 95% core deposits ¹ o Additional scale and synergies will enhance First Interstate’s earnings power o Increases the percentage of First Interstate’s footprint operating in higher growth markets o Financially attractive transaction on both a standalone and combined basis o > 3%+ EPS accretion in first full year and beyond o Tangible Book Value per share earnback of <2.0 years using crossover method Strong Transaction o Economics 20%+ IRR o Significant core deposits and excess liquidity o Continued positive operating leverage with over $14 billion in assets pro forma o All metrics reported inclusive of impact of Durbin Amendment o Strong understanding of markets, diversified lending products and approach to relationship banking o Similar credit cultures focused on conservative underwriting with high quality portfolios Low Execution Risk o Comprehensive due diligence process completed o Retention of key personnel 4 1. Core deposits for IIBK and CMYF excludes jumbo time deposits with balances greater than $100,000; For the quarter ended June 30, 2018.

Overview of Idaho Independent Bank & Community 1st Bank As of June 30, 2018 Idaho Independent Bank Overview Community 1st Bank Overview Headquarters Coeur d’Alene, Idaho Headquarters Post Falls, Idaho Footprint 11 branches Footprint 3 branches Assets $725 million Assets $129 million Gross Loans $362 million Gross Loans $78 million Deposits $610 million Deposits $116 million Total Equity $69 million Total Equity $13 million Regulatory Loan and Deposit Composition Regulatory Loan and Deposit Composition Farm / Ag Other Farm / AgOther CDs Consumer CDs 1-4 Family 0.3% 0.4% Consumer 3.7% 0.4% 2.4% 1.4% 1-4 Family 16.6% 14.6% Transaction 3.6% 15.0% 21.2% C&D C&D Multifamily 19.9% 19.4% Transaction 4.7% Multifamily 42.2% 9.1% Savings & MMDA Savings & MMDA 76.3% C&I C&I CRE 41.2% 22.6% 13.5% 39.5% CRE 31.6% Loans: $362mm Deposits: $610mm Deposits: $116mm Loans: $78mm Cost of Deposits: 0.34% Yield on Loans: 5.85% Cost of Deposits: 0.09% Yield on Loans: 5.86% 5 Financial data per SNL Financial for the quarter ended June 30, 2018

Attractive, Low Cost Pacific Northwest Deposit Markets IIBK and CMYF have strong market share in complementary Pacific Northwest markets FIBK (149) IIBK (11) CMYF (3) Top MSAs - Deposit Market Share Market Deposits Deposits % Proj. Market Growth IIBK Rank Branches ($mm) of PF Total 5-Year CAGR Boise City, ID 8 7 $391 2.8% 1.5% Coeur d'Alene, ID 6 2 $185 1.3% 1.4% Hailey, ID 8 1 $19 0.1% 0.7% CMYF Coeur d'Alene, ID 7 3 $116 0.8% 1.4% 6 Source: S&P Global Market Intelligence, FDIC deposit data as of June 30, 2018

Strong Pro Forma Idaho State Market Share FIBK pro forma is the largest community bank in Boise and the second largest community bank in Coeur d'Alene Total Active Total Deposit Market Branches Total Deposits Market Share YoY Deposit 5-Year Deposit Community Bank in Strong Economies Rank Company Name 2018 2018 ($000) 2018 (%) Growth 2018 (%) CAGR 2018 (%) 1 Wells Fargo & Co. 76 5,627,304 21.70 0.23 4.14 Spokane, WA and Coeur d’Alene, ID Markets 2 U.S. Bancorp 96 4,668,227 18.01 5.04 6.00 3 KeyCorp 26 1,672,224 6.45 2.07 5.17 o Diverse economic region including strong 4 Zions Bancorp. 23 1,527,933 5.89 3.54 2.91 corporate and government employers, and 5 Glacier Bancorp Inc. 24 1,392,467 5.37 9.60 9.19 robust agriculture 6 Pro Forma FIBK / IIBK / CMYF 1,381,845 5.33 -- -- 6 D.L. Evans Bancorp 31 1,228,147 4.74 8.24 6.87 o Spokane Region is the 5th largest aerospace 7 Bank of America Corp. 3 1,058,397 4.08 (5.62) 36.59 cluster in the U.S.1 8 JPMorgan Chase & Co. 20 895,882 3.46 11.96 14.23 9 Bank of Commerce 15 879,024 3.39 4.19 4.41 Greater Boise, ID Market 10 Washington Federal Inc. 24 811,672 3.13 2.27 (10.41) o Boise MSA is the third largest Metro in the 11 W.T.B. Financial Corp. 15 665,032 2.57 10.38 8.92 Northwest 12 First Interstate BancSystem Inc. 18 656,378 2.53 (10.05) (2.90) 13 Idaho Independent Bank 11 609,941 2.35 10.19 9.66 o Boise ranked #1 on Forbes' 2018 list of 25 Community 1st Bank 3 115,526 0.45 12.31 8.95 America's fastest-growing cities o Government, education and healthcare remain core industries, with a large in-flow of technology companies joining the ranks of Hewlett-Packard and Micron Technologies 7 Financial data per SNL Financial 1. Greater Spokane Incorporated 2. Deposit data per FDIC as of June 30, 2018

Top Ranked Idaho Community Banking Franchise Pro Forma¹ Total Assets 500,008 725,016 129,800 1,354,824 Total Loans 479,565 362,049 78,283 919,897 Total Deposits 656,378 609,941 115,526 1,381,845 ID Deposit Market Share % 2.53% 2.35% 0.45% 5.33% Market Rank 12 13 25 6 o Moves First Interstate from #5 bank deposit market share to #3 bank deposit market share in Boise (Ada County) o Moves First Interstate from # 9 bank deposit market share to #4 bank deposit market share in Coeur d’Alene (Kootenai County) 8 Financial data per SNL Financial 1. Financial data as of June 30, 2018 and FIBK pro forma for NBCT acquisition. Combined pro forma column excludes growth and purchase-accounting adjustments.

Diverse, Low Risk Loan Portfolio As of June 30, 2018 Loan Overview Strong Loan Growth $365,891 $361,798 o 9.4% / 9.1% CAGR since 2013 for IIBK and CMYF $324,659 $292,202 $266,052 o 86% / 63% of loans floating with Prime or LIBOR for IIBK and CMYF $241,581 o Expertise in business banking $76,236 $78,283 o 42% / 38% Owner-Occupied CRE and C&I for IIBK and CMYF $52,803 $58,102 $64,876 $66,438 o 0.26% / 0.00% NPAs / Total Assets ratio for IIBK and CMYF 2013 2014 2015 2016 2017 2Q'18 CMYF IIBK First Interstate IIBK CMYF Pro Forma¹ Other Farm / Other Farm / Ag Other 2.5% Ag 0.4% Farm / Ag Other Farm / Ag 5.5% Consumer 1-4 Family Consumer 2.4% 1-4 Family 0.3% 0.4% 1-4 Family 3.7% 3.6% 14.6% 5.4% Consumer 19.3% 1.4% 1-4 Family C&D Consumer 19.0% 12.4% C&D Multifamily 15.0% 19.4% 12.0% Multifamily 19.9% 4.7% Multifamily Multifamily 3.1% C&D 9.1% C&D 3.2% 9.4% 10.0% C&I CRE C&I CRE CRE 22.6% 31.6% CRE C&I 13.5% 39.5% C&I 33.2% 33.4% 14.7% 14.7% Total: $8,473mm Total: $362mm Total: $78mm Total: $8,913mm Yield on Loans: 4.89% Yield on Loans: 5.85% Yield on Loans: 5.86% Yield On Loans: 4.94% 9 Financial data per SNL Financial 1. Pro forma as of 6/30/2018; FIBK pro forma for NBCT acquisition. Pro Forma excludes growth and purchase-accounting adjustments. Note: Loan compositions reflect regulatory data as of June 30, 2018. CAGR calculated from 12/31/13 to 6/30/18

Attractive Low Cost Deposit Base As of June 30, 2018 IIBK Deposit Overview CMYF Deposit Overview Cost of Deposits o 0.09% cost of deposits o 0.34% cost of deposits 0.34% 0.29% 0.28% o 21% transaction deposits o 42% transaction deposits 1 1 o ~99% core deposits o ~86% core deposits 0.09% FIBK IIBK CMYF Pro Forma First Interstate IIBK CMYF Pro Forma3 CDs CDs CDs 2.4% CDs Transaction 11.0% Transaction Transaction 16.6% 10.6% 17.7% 17.3% 21.2% Transaction 42.2% Savings & MMDA Savings & Savings & 76.3% MMDA MMDA Savings & 71.7% 41.2% MMDA 71.6% Total: $10,650mm Total: $610mm Total: $116mm Total: $11,376mm 10 1. Core deposits excludes jumbo time deposits with balances greater than $100,000. 2. Pro forma as of 6/31/2018; excludes growth and purchase-accounting adjustments. Note: Deposit compositions reflect regulatory data as of June 30, 2018 with FIBK pro forma for NBCT. Financial data per SNL Financial

Summary of Deal Terms IIBK CMYF o 0.5 FIBK Class A common shares per IIBK share, o 0.3784 FIBK Class A common shares per CMYF or ~3.9mm FIBK shares in aggregate share, or ~0.5mm FIBK shares in aggregate Consideration and Deal Value1 o Implied purchase price of $22.73 per share o Implied purchase price of $17.20 per share o $181.3mm in aggregate value o $21.5mm in aggregate value o 23.7x Price / MRQ Earnings Per Share o 18.8x Price / MRQ Earnings Per Share 2 2 Standalone o 9.8x Price / 2019E EPS (with cost synergies) o 7.2x Price / 2019E EPS (with cost synergies) Pricing Ratios o 2.49x Tangible Book Value as of June 30, 2018 o 1.55x Tangible Book Value as of June 30, 2018 o 16.9% Core Deposit Premium 3 o 7.4% Core Deposit Premium 3 Combined Metrics Post-Merger o 93.3% FIBK / 6.0% IIBK / 0.7% CMYF Economic Ownership o 22.7x Price / MRQ Earnings o Customary regulatory approvals Combined o 2.39x Tangible Book Value Required Approvals Pricing Ratios o IIBK and CMYF shareholders o 16.3% Core Deposit Premium 3 Anticipated Closing o 2nd Quarter 2019 11 1. Implied purchase price and aggregate value based on FIBK closing price of $45.45 per share on October [5], 2018. 2. P / 2019E EPS multiples include 100% of Cost Savings 3. Core deposits excludes jumbo time deposits with balances greater than $100,000.

Consolidated Financial Impact Financial Impact and Assumptions Consolidated Financial Impact to FIBK EPS Accretion o ~3.6% accretive to EPS in 2020 o Pro Forma TCE / TA of 8.5% o ~1.4% Tangible Book Value dilution per share Projected o Pro Forma Leverage Ratio Combined Pro TBV Impact o <2.0 year Tangible Book Value earnback of 9.4% Forma Capital using crossover method o Pro Forma Total Risk-Based Impact Capital Ratio of 13.0% Internal Rate of o IIBK ~24% and CMYF ~31% Return Assumptions1 IIBK CMYF o 56% of IIBK’s non-interest expense base o 45% of CMYF’s non-interest expense base Synergies o 82% of branches within 1 mile o 100% of branches within 5 miles o No revenue synergies modeled o No revenue synergies modeled One-time Deal Costs o Restructuring charge of ~$16.9mm o Restructuring charge of ~$2.5mm o Total Loan Mark: $12.8mm o Total Loan Mark: $2.1 million Fair Market Value o o and Accounting Other Marks: $5.8mm (positive to equity) Other Marks: $0.5mm (positive to equity) Adjustments o CDI: $14.9mm amortized using sum of years o CDI: $2.4mm amortized using sum of years digits over 10 years digits over 10 years Durbin Impact o $0.6mm decrease in annual interchange revenue o Minimal impact from Durbin Amendment 12 1. Pre-tax amounts.

Comprehensive Due Diligence and Preparation Experienced And o First Interstate management and employees have significant acquisition and integration experience Disciplined o Five successful acquisitions in the last five years Acquisition And o Cascade acquisition already integrated and systems converted, Northwest Bancorporation, Inc. in Integration progress Approach o Integration planning and readiness underway o Comprehensive due diligence process o Extensive credit due diligence, including thorough loan file review and credit re-underwriting, significant local knowledge o Reviewed 69%/62% of Ag, Commercial and Commercial Real Estate loans of IIBK and CMYF Thorough o Reviewed 100% of criticized loans and 100% of OREO Due Diligence o Detailed review of cost structure and expected synergies Process o Identified cost savings opportunities o Conservative earnings expectations o Revenue synergies quantified but not included in model o Thorough review of all regulatory, compliance, legal and operational risks 13

Strengthens Outlook for First Interstate o Strong earnings contribution from IIBK and CMYF fully realized in 2020 o 3%+ Earnings Per Share accretion with conservative assumptions Benefits of o Transaction cost savings result in improved pro forma efficiency ratio Operating o Cross sell opportunities for Wealth Management services, commercial credit cards, indirect & SBA Leverage lending o Significant core deposit base and excess liquidity o Capitalize on First Interstate’s, IIBK’s and CMYF’s investment in people, processes and technology o Complements First Interstate’s existing footprint between the Mountain West and Pacific Northwest Strengthens o Ability to further leverage recent Northwest Bancorporation acquisition and legacy FIBK franchise Existing and o Further expands distribution network into diverse, high deposit growth markets in Pacific Northwest Opens New Markets o Significant opportunity for increased deposit gathering in new markets for Growth o Opportunity to accelerate growth o Geographic and loan portfolio diversification reduces balance sheet risk 14