Claim: Silicon Valley is in California.

San Mateo County -LRB- -LSB- ˌsæn_məˈteɪ . oʊ -RSB- ; Spanish for `` Saint Matthew '' -RRB- is a county located in the U.S. state of California . As of the 2010 census , the population was 718,451 . The county seat is Redwood City .   San Mateo County is included in the San Francisco-Oakland-Hayward , CA Metropolitan Statistical Area , and is part of the San Francisco Bay Area , the nine counties bordering San Francisco Bay . It covers most of the San Francisco Peninsula . San Francisco International Airport is located at the northern end of the county , and Silicon Valley begins at the southern end . The county 's built-up areas are mostly suburban with some areas being very urban , and are home to several corporate campuses .