Good soils, temperate climate, and abundant food and water attracted indigenous peoples to the Sonoma Valley for at least 12,000 years before Spanish missionaries settled in the early 19th century. As many as 5,000 Native Americans lived in what is now Sonoma County at any one time. Local tribes included the Pomo-Kashaya, Wapo, and Patwin.
In 1823, Mission San Francisco Solano de Sonoma was established by Father Junipero Serra. It was the only California mission installed after Mexican independence from Spanish rule. Sonoma was first acknowledged by Mexico as a City in 1835. Mariano Guadalupe Vallejo, a lieutenant later promoted to General, led the transformation of Sonoma into a Mexican pueblo. He oversaw construction of the eight-acre central Plaza (still the largest in California) and the street grid, including the 110–foot wide Broadway. When his nephew, Juan Bautista Alvarado, was named governor of the Mexican state of Alta California in 1838, Vallejo was named military governor of the state.
On June 14, 1846, Sonoma was declared the capital of the “Bear Flag Republic” in a revolt against Mexican control of California. The town’s status as the nominal capital of California lasted 25 days, ending with California’s annexation by the United States. Vallejo supported the Americans when Mexico ceded all of California and the rest of the Southwest to the United States in 1848.
After California achieved statehood in 1850, Vallejo was elected a state senator and lobbied to maintain Sonoma as the county seat; however, Santa Rosa won the honor in an 1854 county election still questioned by some historians. With U.S. rule came the appropriation of many land holdings, and Vallejo lost almost all of his real estate, which once amounted to 7 million acres. His home on West Spain Street was all that remained of his once immense land holdings when he died in 1890. Sonoma was incorporated as a U.S. City in 1883.