The area in which today's San Pablo is situated was originally occupied by the Cuchiyun band of the Ohlone indigenous people. The area was claimed for the king of Spain in the late 1700s and was granted for grazing purposes to the Mission Dolores located in today's San Francisco. Upon Mexico's independence from Spain, church properties were secularized and in 1823, the area became part of a large grant to an ex-soldier stationed at the San Francisco Presidio, Francisco María Castro. The grant was given the name Rancho San Pablo, thus originating the name for today's city as well as for one of the East Bay's oldest principal roads, today's San Pablo Avenue (called in the prior Spanish era "El Camino Real de la Contra Costa").
A historic remnant of the city's Mexican era is preserved as a California State Landmark (No.512): the Alvarado Adobe, constructed in 1842 by one of Francisco Castro's sons, Jesús María Castro, for his mother, Gabriéla Berryessa de Castro. Upon Gabriéla's death in 1851, it was inherited by her daughter, Martina Castro de Alvarado, wife of Juan Bautista Alvarado, who was Governor of California from 1836 to 1842. The Alvarado Adobe was demolished in the mid 20th century to make way for a motel. A reproduction was built later in the century which is located in the San Pablo Civic Center on the northwest corner of San Pablo Avenue and Church Lane.
According to the United States Census Bureau, the city has a total area of 6.7 km² (2.6 mi²), all land.
San Pablo lies on a sedimentary plain between the northern end of the Berkeley Hills and San Pablo Bay, although the city limits do not extend to the bayshore. The city is traversed by two principal creeks, Wildcat Creek and San Pablo Creek, both of which originate in the ranges to its southeast. It is also traversed along its northeast limits by the Hayward Fault, a major branch of the San Andreas Fault which lies to the west.
As of the census of 2000, there were 30,215 people, 9,051 households, and 6,489 families residing in the city. The population density was 4,521.7/km² (11,726.9/mi²). There were 9,340 housing units at an average density of 1,397.7/km² (3,625.0/mi²). The racial makeup of the city was 31.62% White, 18.33% Black or African American, 0.90% Native American, 16.37% Asian, 0.51% Pacific Islander, 25.44% from other races, and 6.83% from two or more races. 44.65% of the population were Hispanic or Latino of any race.
Of the 9,051 households, 44.0% had children under the age of 18 living with them, 44.5% were married couples living together, 19.7% had a female householder with no husband present, and 28.3% were non-families. 22.5% of all households were made up of individuals and 8.1% had someone living alone who was 65 years of age or older. The average household size was 3.29 and the average family size was 3.87.
In the city the population was spread out with 31.7% under the age of 18, 10.9% from 18 to 24, 31.8% from 25 to 44, 16.9% from 45 to 64, and 8.7% who were 65 years of age or older. The median age was 30 years. For every 100 females there were 96.5 males. For every 100 females age 18 and over, there were 93.3 males.
The median income for a household in the city was $37,184, and the median income for a family was $42,042. Males had a median income of $31,599 versus $28,140 for females. The per capita income for the city was $14,303. About 15.5% of families and 18.1% of the population were below the poverty line, including 23.0% of those under age 18 and 11.5% of those age 65 or over.
The languages spoken were 42.49% English, 40.3% Spanish, 5.47% Tagalog, 2.8% Vietnamese, 2.52% Miao-Mien, 1.95% Laotian, 1.18% Punjabi, 0.92% Portuguese, 0.75% Chinese, 0.57% Hindi, and 0.95% other languages spoken by less than 0.5% of the population each.
San Pablo is probably best known by western United States residents as the location of a major Northern California United Parcel Service hub, and thus frequently shows up on their web-based tracking service as a destination (or idling) spot for their package.