Exhibit 99.1

NEWS RELEASE

ARTHUR J. GALLAGHER & CO. ANNOUNCES

FIRST QUARTER 2017 FINANCIAL RESULTS

ROLLING MEADOWS, IL, April 27, 2017 — Arthur J. Gallagher & Co. (NYSE: AJG) today reported its financial results for the quarter ended March 31, 2017. Management will host a webcast conference call to discuss these results on Friday, April 28, 2017 at 9:00 a.m. ET/8:00 a.m. CT. To listen to the call, and for printer-friendly formats of this release and the “Supplemental Quarterly Data” and “CFO Commentary,” which may also be referenced during the call, please visit ajg.com/IR. These documents contain both GAAP and non-GAAP measures. Investors and other users of this information should read carefully the section entitled “Information Regarding Non-GAAP Measures” beginning on page 7.

Summary of Financial Results – First Quarter

Reconciliations of non-GAAP measures begin on page 2

“We had an excellent start to 2017. Coming off a terrific 2016, we delivered solid organic growth, outstanding revenue growth from our tuck-in M&A strategy and strong margin expansion in the first quarter,” said J. Patrick Gallagher, Jr., Chairman, President and CEO. “Our unique culture is thriving around the world, demonstrated by being selected as a World’s Most Ethical Company for the sixth year in a row by the Ethisphere Institute. Our global businesses are working well together, the team is energized and we are successfully executing our strategies. We are well positioned for the remainder of 2017.”

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Quarter Ended March 31 Reported GAAP to Adjusted Non-GAAP Reconciliation:

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Brokerage Segment Reported GAAP to Adjusted Non-GAAP Reconciliations (dollars in millions):

Our domestic and international operations both posted growth similar to the 2.9% growth in organic commission and fee revenues for first quarter 2017.

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Brokerage Segment Reported GAAP to Adjusted Non-GAAP Reconciliations (continued):

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Risk Management Segment Reported GAAP to Adjusted Non-GAAP Reconciliations (dollars in millions):

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Corporate Segment Reported GAAP to Adjusted Non-GAAP Reconciliations (dollars in millions):

Interest and banking costs and debt - At March 31, 2017, Gallagher had $2,450.0 million of borrowings from private placements, $395.0 million of short-term borrowings under its line of credit facility and $104.4 million outstanding under a revolving loan facility that provides funding for premium finance receivables, which are fully collateralized by the underlying premiums held by insurance carriers, and as such are excluded from our debt covenant computations. Gallagher anticipates closing private placements of debt in or around August 2017 to fund the repayment of debt maturing around such time. Any such private placement notes have not been, and will not be, registered under the Securities Act and may not be offered or sold in the United States absent registration or an applicable exemption from registration requirements. This earnings release shall not constitute an offer to sell or the solicitation of an offer to buy the notes.

Clean energy - Consists of the operating results related to our investments in 34 clean coal production plants and royalty income from clean coal licenses related to Chem-Mod LLC. Additional information regarding these results is available in the “CFO Commentary” at ajg.com/IR.

Acquisition costs - Consists mostly of external professional fees and other due diligence costs related to acquisitions.

Corporate - Consists of overhead allocations mostly related to corporate staff compensation and other corporate level activities. In the CFO commentary document as of January 26, 2017, we provided an estimate of our first quarter 2017 after-tax loss for this line ranging between $6.0 million and $7.0 million. As shown above, we actually reported a loss of $3.3 million. The favorable variance occurred because of more income tax benefit from a new accounting standard for employee share-based compensation than we estimated in January 2017.

Litigation Settlement - During the third quarter of 2015, Gallagher settled litigation against certain former U.K. executives and their advisors for a pretax gain of $31.0 million ($22.3 million net of costs and taxes). Incremental expenses that arose in connection with this matter will result in quarterly after-tax charges being incurred through June 30, 2017.

Home Office Lease Termination/Move - During first quarter 2017, we relocated our corporate office headquarters to a nearby suburb of Chicago. Move-related after-tax charges were $2.4 million in the first quarter of 2017. After-tax lease abandonment charges are expected to total approximately $4.0 million and $1.5 million in the second and third quarters of 2017, respectively.

Income Taxes

Gallagher allocates the provision for income taxes to its Brokerage and Risk Management segments using the local country statutory rates. Gallagher’s consolidated effective tax rate for the quarters ended March 31, 2017 and 2016 was (29.6)% and (3.9)%, respectively, which was lower than the statutory rate due to the amount of IRC Section 45 tax credits earned and the impact of the income tax benefit of stock compensation in first quarter 2017.

At-the-market equity program - Gallagher has an at-the-market equity program under which up to $15.6 million of its common stock remaining in the program may be sold through Morgan Stanley & Co. LLC as sales agent. During first quarter 2017, Gallagher did not sell any shares of its common stock under the program.

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Webcast Conference Call

Gallagher will host a webcast conference call on Friday, April 28, 2017 at 9:00 a.m. ET/8:00 a.m. CT. To listen to this call, please go to ajg.com/IR. The call will be available for replay at such website for at least 90 days.

About Arthur J. Gallagher & Co.

Arthur J. Gallagher & Co., an international insurance brokerage and risk management services firm, is headquartered in Rolling Meadows, Illinois, has operations in 33 countries and offers client-service capabilities in more than 150 countries around the world through a network of correspondent brokers and consultants.

Cautionary Information

This press release contains “forward-looking statements” within the meaning of the Private Securities Litigation Reform Act of 1995. When used in this press release, the words “anticipates,” “believes,” “contemplates,” “see,” “should,” “could,” “will,” “estimates,” “expects,” “intends,” “plans” and variations thereof and similar expressions, are intended to identify forward-looking statements. Examples of forward-looking statements include, but are not limited to, statements regarding (i) the amount of, and potential uses for, investment returns generated by Gallagher’s clean energy investments; (ii) our corporate income tax rate; (iii) anticipated future results or performance of any segment or the Company as a whole; (iv) the premium rate environment; (v) the economic environment; (vi) anticipated acquisition integration costs; and (vii) anticipated actions to be taken in connection with maturing debt.

Gallagher’s actual results may differ materially from those contemplated by the forward-looking statements. Readers are therefore cautioned against relying on any of the forward-looking statements, which are neither statements of historical fact nor guarantees or assurances of future performance. Important factors that could cause actual results to differ materially from those in the forward-looking statements include the following:

Please refer to Gallagher’s filings with the SEC, including Item 1A, “Risk Factors,” of its Annual Report on Form 10-K for the fiscal year ended December 31, 2016 for a more detailed discussion of these and other factors that could impact its forward-looking statements. Any forward-looking statement made by Gallagher in this press release speaks only as of the date on which it is made. Except as required by applicable law, Gallagher does not undertake to update the information included herein or the corresponding earnings release posted on Gallagher’s website.

Information Regarding Non-GAAP Measures

In addition to reporting financial results in accordance with GAAP, this press release provides information regarding EBITDAC, EBITDAC margin, adjusted EBITDAC, adjusted EBITDAC margin, diluted net earnings per share, as adjusted (adjusted EPS), for the Brokerage and Risk Management segments, adjusted revenues, adjusted compensation and operating expenses, adjusted compensation expense ratio, adjusted operating expense ratio and organic revenue measures for each operating segment. These measures are not in accordance with, or an alternative to, the GAAP information provided in this press release. Gallagher’s management believes that these presentations provide useful information to management, analysts and investors regarding financial and business trends relating to Gallagher’s results of operations and financial condition. See further below for definitions and the reason each of these measures is useful to investors. Gallagher’s industry peers may provide similar supplemental non-GAAP information with respect to one or more of these measures, although they may not use the same or comparable terminology and may not make identical adjustments. The non-GAAP information provided by Gallagher should be used in addition to, but not as a substitute for, the GAAP information provided. As disclosed in its most recent Proxy Statement, beginning in first quarter 2017, Gallagher makes determinations regarding certain elements of executive officer compensation, performance share awards and annual cash incentive awards, partly on the basis of measures related to adjusted EBITDAC. Certain reclassifications have been made to the prior year amounts reported in this press release in order to conform them to the current year presentation.

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Adjusted Non-GAAP presentation - Gallagher believes that the adjusted non-GAAP presentations of the current and prior year information, presented in this earnings release, provide stockholders and other interested persons with useful information regarding certain financial metrics of Gallagher that may assist such persons in analyzing Gallagher’s operating results as they develop a future earnings outlook for Gallagher. The after-tax amounts related to the adjustments were computed using the normalized effective tax rate for each respective period. See page 12 for a reconciliation of the adjustments made to income taxes.

Non-GAAP Earnings Measures

Organic Revenues (a non-GAAP measure) - For the Brokerage segment, organic change in base commission and fee revenues excludes the first twelve months of net commission and fee revenues generated from acquisitions and the net commission and fee revenues related to operations disposed of in each year presented. These commissions and fees are excluded from organic revenues in order to help interested persons analyze the revenue growth associated with the operations that were a part of Gallagher in both the current and prior year. In addition, change in base commission and fee revenue organic growth excludes the period-over-period impact of foreign currency translation. For the Risk Management segment, organic change in fee revenues excludes the first twelve months of fee revenues generated from acquisitions and the fee revenues related to operations disposed of in each year presented. In addition, change in organic growth excludes the period-over-period impact of foreign currency translation to improve the comparability of our results between periods by eliminating the impact of the items that have a high degree of variability or are due to the limited-time nature of these revenue sources.

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These revenue items are excluded from organic revenues in order to determine a comparable, but non-GAAP, measurement of revenue growth that is associated with the revenue sources that are expected to continue in the current year and beyond. Gallagher has historically viewed organic revenue growth as an important indicator when assessing and evaluating the performance of its Brokerage and Risk Management segments. Gallagher also believes that using this non-GAAP measure allows readers of our financial statements to measure, analyze and compare the growth from its Brokerage and Risk Management segments in a meaningful and consistent manner.

Reconciliation of Non-GAAP Information Presented to GAAP Measures - This press release includes tabular reconciliations to the most comparable GAAP measures, as follows: for EBITDAC (on pages 10 and 11), for adjusted revenues, adjusted EBITDAC and adjusted diluted net earnings per share (on page 2), for organic revenue measures (on pages 3 and 5, respectively, for the Brokerage and Risk Management segments), for adjusted compensation and operating expenses and adjusted EBITDAC margin (on pages 4 and 5, respectively, for the Brokerage and Risk Management segments). Reported compensation and operating expense ratios can also be found in the “Supplemental Quarterly Data” available at ajg.com/IR.

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Arthur J. Gallagher & Co.

Reported Statement of Earnings and EBITDAC - 1st Qtr Ended March 31,

(Unaudited - in millions except per share, percentage and workforce data)

See “Information Regarding Non-GAAP Measures” on page 7 of 12.

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Arthur J. Gallagher & Co.

Reported Statement of Earnings and EBITDAC - 1st Qtr Ended March 31,

(Unaudited - in millions except share and per share data)

Arthur J. Gallagher & Co.

Consolidated Balance Sheet

(Unaudited - in millions except per share data)

See “Information Regarding Non-GAAP Measures” on page 7 of 12.

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Arthur J. Gallagher & Co.

Other Information

(Unaudited - data is rounded where indicated)

Reconciliation of Non-GAAP Measures - Pre-tax Earnings and Diluted Net Earnings per Share (Unaudited)

(Unaudited - in millions except share and per share data)