Sonoma County is the northernmost of the nine San Francisco Bay Area counties and the one with the largest land mass.

The Pomo, Coast Miwok and Wappo peoples were the earliest human settlers of Sonoma County, between 8000 and 5000 BC.

The Russians were the first Europeans to settle in Sonoma County, with the Russian-American Company establishing Fort Ross on the Sonoma coast in 1812.

The Mission San Francisco Solano, founded in 1823 as the last and northernmost of twenty-one California missions, is in the present City of Sonoma at the northern end of El Camino Real.

El Presidio de Sonoma was established in 1836 by Comandante General Mariano Guadalupe Vallejo. His duties included keeping an eye on the Russian traders at Fort Ross, secularizing the Mission, maintaining cooperation with the Native Americans of the entire region, and doling out the lands for large estates and ranches.

The City of Sonoma was the site of the Bear Flag Revolt in 1846.

Sonoma was one of the original counties formed when California became a state in 1850.