In 1795, Governor Diego de Borica gave José Darío Argüello a Spanish land grant known as Rancho de las Pulgas. This rancho was the largest grant on the peninsula consisting of 35,260 acres (142.7 km2).
As a local geographic term, the area referred to as “The Peninsula” is distinct from that denoted by “The City”, and refers to the portion south of San Francisco. The appellation may date to the period, prior to 1856, when the City of San Francisco and the County of San Francisco were separate entities, the latter then coextensive with contemporary San Mateo County and San Francisco City-County. The City-County owns several disjunct properties along the whole of the Peninsula (mostly water pumping stations connected to the Hetch Hetchy Valley on which San Francisco has a permanent leasehold); thus, most of the larger communities in San Mateo County are de facto suburbs of San Francisco, with the neighboring communities of Pacifica, Daly City, Broadmoor, Colma, South San Francisco, Half Moon Bay, San Bruno, and Brisbane being immediate suburbs. The remaining suburban area of the Peninsula is on the east side of the Santa Cruz Mountains, along San Francisco Bay; the west and south-central portions of the Peninsula are mostly rural, unincorporated and unorganised areas.
A substantial portion of Silicon Valley is located on the peninsula. In Silicon Valley are the headquarters of some of the largest tech companies in the world, such as Google, Yahoo, Facebook and Apple. Over the last decade or so there has been an influx of immigration into the Bay Area from places like India and China to work in the technology industry. There are well over 6,600 tech startups in the Valley and new ones are created every day.
The east side of the peninsula is a densely populated and largely urban and suburban area that includes portions of Silicon Valley. It forms a commuter area between San Francisco to the north and San Jose to the south. A number of major thoroughfares run north-south: El Camino Real (SR 82) and US 101 on the east side along the bay, Interstate 280 down the center, Skyline Boulevard (SR 35) along the crest of the Santa Cruz Mountains, and Highway 1 on the west along the Pacific. The Caltrain commuter rail line runs roughly parallel to the El Camino Real (State Route 82) and Highway 101 corridors.
The bridges in the Peninsula include the Dumbarton Bridge, the San Francisco – Oakland Bay Bridge, the San Mateo-Hayward Bridge and the Golden Gate Bridge.
Along the center line of the Peninsula is the northern half of the Santa Cruz Mountains, formed by the action of plate tectonics along the San Andreas Fault. In the middle of the Peninsula along the fault is the Crystal Springs Reservoir. Just north of the Crystal Springs reservoir is San Andreas Lake, after which the geologic fault was originally named.
The San Francisco Peninsula contains a variety of habitats including estuarine, marine, oak woodland, redwood forest, coastal scrub and oak savanna. There are numerous species of wildlife present, especially along the San Francisco Bay estuarine shoreline, San Bruno Mountain, Fitzgerald Marine Reserve and the forests on the Montara Mountain block.
The county is home to several endangered species including the San Francisco garter snake, the Mission blue butterfly and the San Bruno elfin butterfly, all of which are endemic to San Mateo County. The endangered California clapper rail is also found on the shores of San Francisco Bay, in the cities of Belmont and San Mateo.
A number of noteworthy parks and nature preserves are found on the San Francisco Peninsula, including:
- Edgewood Park, San Mateo County
- Golden Gate National Recreation Area – several units are located on the Peninsula
- Midpeninsula Regional Open Space District – several preserves
- Sanborn Park, Santa Clara County