Solano County is a county located in the U.S. state of California. As of the 2010 census, the population was 413,344. The county seat is Fairfield. Solano County comprises the Vallejo-Fairfield, CA Metropolitan Statistical Area, which is also included in the San Jose-San Francisco-Oakland, CA Combined Statistical Area. Solano County is the northeastern county in the nine-county San Francisco Bay Area region.