Marsh & McLennan Companies, Inc. is a global professional services firm, headquartered in New York City with businesses in insurance brokerage, risk management, reinsurance services, talent management, investment advisory, and management consulting. Its four main operating companies are Guy Carpenter, Marsh, Mercer, and Oliver Wyman Group.

Marsh & McLennan Companies (MMC) is a professional services firm that’s behind some of today’s major insurance and consulting players, including Marsh, Guy Carpenter, Mercer, and Oliver Wyman. The company was founded by Henry W. Marsh and Donald R. McLennan, who merged their two firms in 1904 to create the largest insurance agency in the world.

Before the merger, Marsh had seen an opportunity for insurance brokers to distribute risks across firms following the Great Fire of 1871, which wiped out many insurers.

Marsh & McLennan Companies ranked #212 on the 2018 Fortune 500 ranking, the company's 24th year on the annual Fortune list, and #458 on the 2017 Forbes Global 2000 List.  Marsh & McLennan's 2016 revenue of $13.2 billion ranked it #1 on Business Insurance's ranking of the world's largest insurance brokers.

In April 2019, MMC completed a US$5.6 billion acquisition of Jardine Lloyd Thompson Group plc (JLT). Already the largest insurance broker in the world, Marsh advises around 95% of Fortune 1000 companies. 

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