As of the census , the population was , The county covers most of the San Francisco Peninsula. The county is predominantly suburban and is home to many corporate campuses. San Mateo County was formed in upon the division of San Francisco County, one of the state's 18 original counties established at California statehood in In , the California state government divided the county.
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. The 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.
Although the formation bill named Redwood City the county seat, a May 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. As a place name, San Mateo appears as early as in the diaries of Anza and Font. Until about , the name appeared as San Matheo. These students had to work for their housing and food before classes and in the evenings. Most of them had a good educational background from their homelands, but their lack of knowledge of the English language made it difficult for them to find other jobs in the beginning.
According to the U. A number of bayside watercourses drain the eastern part of the county including San Bruno Creek and Colma Creek. These streams originate along the northern spur of the Santa Cruz Mountains that run through the county.
The northern and eastern parts of the county are very heavy densely populated with largely urban and suburban areas, with many of its cities as edge-cities for the Bay Area, while the deep south and the west-central parts of the county are less densely populated with more rural environment and coastal beaches areas.
