High Quality Content by WIKIPEDIA articles! 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 2005 the population was 712,462. The county seat is Redwood City. It is among the 20 most affluent counties in the United States, in terms of personal, per capita and household income. It is strongly Democratic and ethnically diverse. The county's built-up areas are mostly suburban, and are home to several corporate campuses.