San Mateo County, California

San Mateo County is a county in California, United States, on the San Francisco Peninsula directly south of San Francisco. It had a population of 764,442 at the 2020 census, an estimated 743,568 by 2025, and covers 741 square miles (1,919 km2) centred near 37°26′N 122°22′W.

Redwood City is the county seat; its postal code is 94063. The county holds 16 incorporated cities and 4 incorporated towns, plus about a dozen unincorporated communities with no local government of their own, and it runs on Pacific Time. Its FIPS code is 06081.

Towns & Villages in San Mateo County

San Mateo County has 20 incorporated municipalities — 16 use the “city” designation and 4 (Atherton, Hillsborough, Portola Valley and Woodside) are incorporated as towns, a distinct legal status under California law carrying the same governing powers as a city. The table below lists all 20, ranked by population.

San Mateo County also has eleven census-designated places (CDPs) — unincorporated communities with a fixed census boundary but no city government — ranked below by their 2020 population.

One CDP split at the 2020 census: the former Highlands-Baywood Park, between Belmont and San Carlos, is now counted as two separate places, Highlands and Baywood Park, both without a published population total of their own. Kings Mountain and Skyline, two long-used names for rural hill hamlets along Skyline Boulevard, have no census boundary and are not counted separately.

ZIP Codes

• 94401–94403 — San Mateo (94404 shared with Foster City)
• 94010 — Burlingame (also serves Hillsborough)
• 94005 — Brisbane
• 94014–94015 — Daly City (also serves Colma and Broadmoor)
• 94018 — El Granada
• 94019 — Half Moon Bay
• 94020 — La Honda
• 94021 — Loma Mar
• 94025 — Menlo Park (also serves North Fair Oaks and West Menlo Park)
• 94027 — Atherton
• 94028 — Portola Valley (also serves Ladera)
• 94030 — Millbrae
• 94037 — Montara
• 94038 — Moss Beach
• 94044 — Pacifica
• 94060 — Pescadero
• 94061–94065 — Redwood City (94062 also serves Woodside and Emerald Lake Hills)
• 94066 — San Bruno
• 94070 — San Carlos
• 94080 — South San Francisco
• 94303 — East Palo Alto

Broadmoor and Highlands-Baywood Park have no ZIP of their own; mail there is delivered under the neighboring incorporated city’s code.

Geography

San Mateo County occupies the base of the San Francisco Peninsula, with San Francisco Bay on its northeast side and the Pacific Ocean on its west. Of its 741 square miles, 448 are land and 293 are water — the bay accounts for nearly 40 percent of the county’s total area, an unusually high share driven by San Francisco International Airport’s bayfill site and the shallow tidal flats off Redwood City and Foster City. The Santa Cruz Mountains run down the county’s spine, separating a dense bayside strip of cities from a rural, fog-belt coastline known locally as the Coastside.

San Francisco County borders the county to the north, Santa Clara County to the south, and Santa Cruz County to the southwest; Alameda County lies across San Francisco Bay to the east, linked by the San Mateo–Hayward Bridge and the Dumbarton Bridge. San Francisco County, Santa Clara County, Santa Cruz County and Alameda County all border San Mateo County directly or across the bay.

Demographics

San Mateo County had a population of 764,442 at the 2020 census, and the Census Bureau’s latest estimate puts the county near 743,568 residents by 2025, a slight decline typical of Bay Area counties since the pandemic. San Mateo and Daly City are the two largest cities, together holding more than a quarter of the county’s population. Median household income runs $158,855, well above California’s statewide figure of $95,521. Population density works out to roughly 1,707 people per square mile of land, among the densest in California outside San Francisco itself.

Economy & Local Life

Professional, scientific and technical services employ the most workers in the county, about 68,410 people, followed by health care and social assistance at 51,272 and retail trade at 33,301. San Francisco International Airport (SFO), one of the busiest airports in the United States, sits on unincorporated land near Millbrae and San Bruno and is itself among the county’s largest employers. South San Francisco calls itself the birthplace of biotechnology — Genentech, founded in 1976 and still headquartered there, anchors a biotech cluster that has since grown to dozens of drug and life-sciences companies. Foster City is home to Visa’s global headquarters and to Gilead Sciences; Redwood City hosts Electronic Arts and Box; San Mateo is headquarters to GoPro and Sony Interactive Entertainment; and San Bruno is home to YouTube’s headquarters. The county’s biggest annual event is the Half Moon Bay Art & Pumpkin Festival each October, which draws crowds well beyond the Coastside’s own population.

History

The Ohlone people lived along the peninsula and coast for thousands of years before Spanish missionaries and soldiers arrived in the 1770s; the Rancho de las Pulgas land grant later covered much of the county’s bayside. California’s legislature formed San Mateo County on 19 April 1856, dividing it out of San Francisco County along a line north of San Bruno Mountain, and organized county government on 18 April 1857. The name comes from the Spanish for Saint Matthew, in use as a place name in the area since at least 1776.

The county grew as a rail-served retreat for San Francisco’s wealthy in the late 1800s — Atherton and Hillsborough both trace their large-lot character to that era — and expanded rapidly after the 1906 San Francisco earthquake sent refugees south into Daly City and other bayside towns. San Francisco International Airport opened in 1927 on filled bayland in unincorporated San Mateo County. Genentech’s 1976 founding in South San Francisco is widely credited with starting the modern biotechnology industry.

Getting There

US 101 runs the length of the county along the bay, Interstate 280 follows the Santa Cruz Mountains on the western side, Interstate 380 connects the two near the airport, and State Route 1 traces the Pacific coast. State Route 92 is the main east-west link, crossing the bay on the San Mateo–Hayward Bridge toward Half Moon Bay in the other direction.

• San Francisco — about 20 miles north
• San Jose — about 20 miles south
• Oakland — about 20 miles east, across the bay
• Santa Cruz — about 35 miles southwest

San Francisco International Airport, in unincorporated San Mateo County near Millbrae, is the county’s own commercial airport and one of the busiest in the country. Caltrain runs the length of the county with twelve stations between Bayshore and Redwood City, connecting south to San Jose Diridon station, about 20 miles away, for Amtrak’s Capitol Corridor and Coast Starlight service.

Frequently Asked Questions

Where is San Mateo County located?
It occupies the San Francisco Peninsula directly south of San Francisco, bordered by Santa Clara, Santa Cruz and, across the bay, Alameda counties.

What is the population of San Mateo County?
764,442 at the 2020 census, with a 2025 estimate near 743,568.

How big is San Mateo County?
741 square miles (1,919 km2) total, of which 448 square miles are land and 293 are water.

What is the county seat of San Mateo County?
Redwood City, organized as county government in 1857, is the county seat.

How many towns and cities are in San Mateo County?
20 incorporated places — 16 cities and 4 towns (Atherton, Hillsborough, Portola Valley and Woodside) — plus eleven CDPs such as North Fair Oaks and El Granada.

What ZIP codes does San Mateo County use?
More than two dozen, from 94002 in Belmont to 94303 in East Palo Alto; San Mateo alone uses four, 94401 through 94404.

What is San Mateo County known for?
San Francisco International Airport, Genentech and the biotech cluster it started in South San Francisco, and the tech headquarters of Visa, YouTube and GoPro.

More counties in this state: Towns and villages in California.