San Mateo County, California was formed on 19 April 1856 from Santa Cruz and San Francisco Counties, California.
San Mateo County is a county located in the San Francisco Bay Area of the U.S. state of California. It covers most of the San Francisco Peninsula just south of San Francisco and north of Santa Clara County. San Francisco International Airport is located at the northern end of the county, and Silicon Valley begins at the southern end. As of 2000 the population was 707,161. The county seat is Redwood City.
The county bears the Spanish name for Saint Matthew. As a place name, Saint Matthew appears as early as 1776, and the arroyo, the point and the settlement at the unofficial San Mateo Mission are all so designated on the early maps. Until about 1850, the name appeared as San Matheo.