- Total 744 sq mi (1,930 km2)
- Land 448 sq mi (1,160 km2)
- Water 293 sq mi (760 km2)
Highest elevation 2,603 ft (793 m)
Population (April 1, 2010)
- Total 718,451
- Estimate (2016) 764,797
- Density 970/sq mi (370/km2)
San Mateo County was formed in 1856 after San Francisco County, one of the state’s 18 original counties since California’s statehood in 1850, was split apart. Until 1856, San Francisco‘s city limits extended west to Divisadero Street and Castro Street, and south to 20th Street. In response to the lawlessness and vigilantism that escalated rapidly between 1855 and 1856, the California government decided to divide the county. A straight line was then drawn across the tip of the San Francisco Peninsula just north of San Bruno Mountain. Everything south of the line became the new San Mateo County while everything north of the line became the new consolidated City and County of San Francisco, to date the only consolidated city-county in California. The consolidated city-county of San Francisco was formed by a bill introduced by Horace Hawes, signed by the governor on 19 April 1856. San Mateo County was officially organized on 18 April 1857 under a bill introduced by Senator T.G. Phelps. The 1857 bill defined the southern boundary of San Mateo County as following the south branch of San Francisquito Creek to its source in the Santa Cruz Mountains and thence due west to the Pacific Ocean, and named Redwood City as the county seat. San Mateo County then annexed part of northern Santa Cruz County in March 1868, including Pescadero and Pigeon Point.
Although the forming bill named Redwood City the county seat, a May 1856 election marked by “unblushing frauds … perpetuated on an unorganized and wholly unprotected community by thugs and ballot stuffers from San Francisco” named Belmont the county seat. The election results were declared illegal and the county government was moved to Redwood City, with land being donated from the original Pulgas Grant for the county government on 27 February 1858. Redwood City’s status as county seat was upheld in two succeeding elections in May 1861 and 9 December 1873, defeating San Mateo and Belmont. Another election in May 1874 named San Mateo the county seat, but the state supreme court overturned that election on 24 February 1875 and the county seat has been in Redwood City since.
San Mateo County bears the Spanish name for Saint Matthew. As a place name, San Mateo appears as early as 1776 and several local geographic features were also designated San Mateo on early maps including variously: a settlement, an arroyo, a headland jutting into the Pacific (Point Montara), and a large land holding (Rancho San Mateo). Until about 1850, the name appeared as San Matheo.