About San Mateo County

San Mateo County (/ˌsæn məˈteɪ.oʊ/ san mə-tay-oh; Spanish for "Saint Matthew") 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 2010 the population was 718,451. The county seat is Redwood City. It is ethnically diverse, and affluent. The county's built-up areas are mostly suburban with some areas being very urban, and are home to several corporate campuses.

San Mateo County was formed in 1856 after San Francisco County, one of 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. San Mateo County then annexed part of northern Santa Cruz County in 1868.

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.

San Mateo County straddles the San Francisco Peninsula, with the Santa Cruz Mountains running its entire length. The county encompasses a variety of habitats including estuarine, marine, oak woodland, redwood forest, coastal scrub and oak savannah. There are numerous species of wildlife present, especially along the San Francisco Bay estuarine shoreline, San Bruno Mountain, Fitzgerald Marine Reserve and the forests on the Montara Mountain block. Several creeks discharge to the San Francisco Bay including San Mateo Creek and Laurel Creek and several coastal streams discharge to the Pacific Ocean such as Frenchmans Creek and San Vicente Creek.

The county is home to several endangered species including the San Francisco garter snake and the San Bruno elfin butterfly, both of which are endemic to San Mateo County. The endangered California clapper rail is also found on the shores of San Francisco Bay, in the cities of Belmont and San Mateo. The endangered wildflower Hickman's potentilla is found near the Pacific Ocean on the lower slopes of Montara Mountain. The endangered wildflowers White-rayed pentachaeta, Pentachaeta bellidiflora, San Mateo Woolly Sunflower, Eriophyllum latilobum, Marin Dwarf Flax, Hesperolinon congestum and the San Mateo Thornmint, Acanthomintha duttonii, are found in the vicinity of the Crystal Springs Reservoir.