John Marsh (1799–1856) was a physician, ranchero, and early settler in Mexican California who played a pivotal role in encouraging American immigration and statehood. Born in Danvers, Massachusetts, he studied at Phillips Academy and Harvard before moving west. He worked as a teacher, Indian agent, and trader in the Midwest, where he fathered a son, Charles, with a Lakota-French woman, Marguerite Decouteaux.

In 1836, Marsh traveled to California, became a Mexican citizen, converted to Catholicism, and purchased Rancho Los Méganos in present-day Contra Costa County. Known as the first person to practice Western medicine in California, he charged high fees but earned wealth as both doctor and cattle rancher. Marsh promoted American settlement by writing letters describing the land’s potential, helping spark migration along “Marsh’s Route.” He also participated in political events that weakened Mexican rule and paved the way for U.S. annexation.

Wealthy but unpopular with many workers, Marsh built a large stone mansion near Brentwood. He married Abigail Tuck in 1851 and had a daughter, Alice. In 1856, Marsh was murdered by his employees in a wage dispute. Despite his turbulent life, he is remembered as an influential, if controversial, figure in California history. Marsh Creek, Marsh Creek State Park, and the John Marsh Heritage Highway bear his name.