Marsh McLennan History: Founding, Growth & Key Milestones

The storied origins of Marsh McLennan trace back to one of the most defining catastrophes in American urban history: the Great Chicago Fire of October 1871. In the embers of the devastated metropolis, Henry W. Marsh joined the insurance industry, recognizing that industrializing America required institutional insurance broking rather than local retail policy salesmen. In 1904, Marsh joined forces with Donald R. McLennan—who had built a dominant brokerage insuring Midwestern rail freight—to form Marsh & McLennan in Chicago. The merged firm immediately revolutionized commercial insurance by securing the exclusive broker mandate for the massive Burlington Northern Railroad, creating the concept of national corporate risk management. Over the 20th century, the firm expanded internationally, invented modern reinsurance broking with the acquisition of Guy Carpenter in 1923, launched employee benefits consulting giant Mercer in 1945, and acquired management consulting powerhouse Oliver Wyman in 2003. In 2019, Marsh McLennan acquired London-based broker Jardine Lloyd Thompson (JLT) for $5.6 billion, consolidating its lead as the largest broker on earth.