During the Mexican-American war, areas of Marin County were seized by Americans as part of the conquest of California (1846–1847). Marin County is one of the original 27 counties of California, created February 18, 1850, following adoption of the California Constitution of 1849 and just months before the state was admitted to the Union.

According to General Mariano Vallejo, who headed an 1850 committee to name California’s counties, the county was named for Marin,” great chief of the tribe Licatiut.” Marin had been named “Huicmuse” until he was baptized as “Marino” at about age 20. Marin / Marino was born into the Huimen people, a Coast Miwok tribe of Native Americans who inhabited the San Rafael area. Vallejo believed that “Chief Marin” had waged several fierce battles against the Spanish. Marino definitely did reside at Mission Dolores (in modern San Francisco) much of the time from his 1801 baptism and marriage until 1817, frequently serving as a baptism witness and godfather; he may have escaped and been recaptured at some point during that time. Starting in 1817, he served as an alcalde (in effect, an overseer) at the San Rafael Mission, where he lived from 1817 off and on until his death. In 1821, Marino served as an expedition guide for the Spanish for a couple of years before escaping and hiding out for some months in the tiny Marin Islands (also named after him); his recapture resulted in a yearlong incarceration at the Presidio before his return to the Mission San Rafael area for about 15 years until his death in 1839. In 2009, a plaque commemorating Chief Marin was placed in Mill Valley.