The 1857 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.

[12][11] Although the formation bill named Redwood City the county seat, a May 1856 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.

[13] Redwood City's status as county seat was upheld in two successive elections in May 1861 and December 9, 1873, defeating San Mateo and Belmont.

The Japanese first arrived in San Mateo County and were part of a group guided by Ambassador Tomomi Iwakura in 1872.

[15] A number of male Japanese students came to San Mateo to learn English and many other helpful skills to bring back to Japan.

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.

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.

[19][20] The three-year-old female flew more than 100 miles (160 km) north from Pinnacles National Park, in San Benito County, on May 30, and landed on a private, forested property near Pescadero, on the San Mateo County Coast, where it was photographed by a motion-activated wildlife camera.

[22] Tule elk (Cervus canadensis nannodes) were native to San Mateo County and among the "favored foods" of the Ohlone people based on ethnohistoric and archeological evidence there.

[26] Laguna Alta lay just south of the Interstate 280 and Skyline Boulevard intersection, east of Mussel Rock.

[27] The California Academy of Sciences also has an elk skull fragment collected one mile inland from the mouth of Purisima Creek in 1951.

[32]: 12  The County Superintendent of Schools, Roy Cloud, had visited a one-room schoolhouse in Harrison Canyon as part of his duties; at the time, it was thickly forested with old-growth redwoods and he was alarmed when he learned they were scheduled to be logged.

[32]: 9–11  The Recreation Commission was not created until 1932, and the park was improved as part of the Works Progress Administration efforts starting in 1935.

The racial makeup of San Mateo County was 383,535 (53.4%) White, 20,436 (2.8%) African American, 3,306 (0.5%) Native American, 178,118 (24.8%) Asian (9.8% Filipino, 9.0% Chinese, 1.9% Indian, 1.2% Japanese, 0.8% Korean, 0.5% Vietnamese, 0.3% Burmese, 0.1% Pakistani), 10,317 (1.4%) Pacific Islander (0.6% Tongan, 0.3% Samoan, 0.2% Fijian, 0.1% Native Hawaiian), 84,529 (11.8%) from other races, and 38,210 (5.3%) from two or more races.

Hispanic or Latino of any race were 182,502 persons (25.4%); 15.7% of San Mateo County is Mexican, 2.7% Salvadoran, 1.2% Guatemalan, 1.2% Nicaraguan, 0.7% Peruvian, 0.6% Puerto Rican, 0.2% Colombian, and 0.2% Cuban.

On November 6, 2012, Measure B passed[75] to amend the San Mateo County Charter so that each member of the Board of Supervisors would be elected by voters in his or her district.

On November 4, 2008, San Mateo County voted 61.8% against Proposition 8, which amended the California Constitution to ban same-sex marriages.

A July 2013 Wall Street Journal article identified the Facebook initial public offering (IPO) as the cause of a change in the U.S.' national economic statistics, as San Mateo County—the home of the company—became the top wage-earning county in the country after the fourth quarter of 2012.

[88] Additionally, San Mateo County hosts the headquarters of Visa Inc, Sony Interactive Entertainment, Electronic Arts, YouTube, Genentech, GoPro, and Gilead Sciences, as well as a hub of venture capital firms in Menlo Park and several other technology-related companies.

Some students in San Mateo County's public schools attend outdoor education in La Honda.

San Mateo Outdoor Education is a residential school that teaches major concepts of ecology via exploration of forest, pond, garden, tidepool, wetland, and sandy shore habitats.

Bay Area Rapid Transit (BART) trains serve San Francisco International Airport (SFO) and the northern portion of the county, paralleling El Camino Real / Mission Boulevard at six stations between Daly City and Millbrae, including the station at SFO.

There are many free shuttles that operate from Caltrain, BART, and Ferry stations along fixed routes to local employers during weekday commuting hours (6–10 am and 3–7 pm).

A free shuttle is provided to connect ferry passengers to the South San Francisco Caltrain station, via several business parks on Oyster Point.