According to the United States Census Bureau, the CDP has a total area of 2.8 square miles (7.2 km²), of which, 2.8 square miles (7.2 km²) is land and 0.36% is water.
As of the census of 2000, there were 21,898 people, 7,500 households, and 5,677 families residing in the CDP. The population density was 7,893.4 people per square mile (3,052.3/km²). There were 7,609 housing units at an average density of 2,742.7/sq mi (1,060.6/km²). The racial makeup of the CDP was 63.32% White, 2.81% Black or African American, 0.89% Native American, 15.48% Asian, 0.48% Pacific Islander, 10.82% from other races, and 6.21% from two or more races. 24.65% of the population were Hispanic or Latino of any race. The Hispanic population has been growing rapidly in recent years.
There were 7,500 households, out of which 34.7% had children under the age of 18 living with them, 58.1% were married couples living together, 12.6% had a female householder with no husband present, and 24.3% were non-families. 19.1% of all households were made up of individuals and 11.5% had someone living alone who was 65 years of age or older. The average household size was 2.92 and the average family size was 3.34.
In the CDP the population was spread out with 25.2% under the age of 18, 8.0% from 18 to 24, 29.5% from 25 to 44, 21.3% from 45 to 64, and 16.0% who were 65 years of age or older. The median age was 38 years. For every 100 females there were 94.0 males. For every 100 females age 18 and over, there were 91.3 males.
The median income for a household in the CDP was $66,170, and the median income for a family was $71,787. Males had a median income of $53,626 versus $39,531 for females. The per capita income for the CDP was $21,922. About 3.7% of families and 5.4% of the population were below the poverty line, including 6.4% of those under age 18 and 4.2% of those age 65 or over.
San Lorenzo is an unincorporated community and thus is governed directly by the County of Alameda.
San Lorenzo was part of a large Spanish land grant before the United States' takeover of California in 1848. San Lorenzo was mostly farmland from the mid-19th century to the mid-20th century, a significant center of production of fruit and flowers.
Many of the original inhabitants have been laid to rest in Pioneer Cemetery, including Moses Wicks, who brought oysters to San Leandro Bay (by boat around the cape) from Patchogue, Long Island..
In 1944, under contract to the U.S. Navy, David Bohannon's Greenwood Company began construction of San Lorenzo Village, a tract of two- and three-bedroom homes for workers in the East Bay's war industries. San Lorenzo Village was one of the nation's first planned communities, with parcels designated for schools, churches, parks, and several retail centers. Bohannon's pioneering pre-cutting techniques, referred to as the "California method," were used in later developments, such as the more famous Levittown, Pennsylvania. Home construction continued into the 1950s to accommodate the region's booming population.
San Lorenzo is served by the San Lorenzo Unified School District, which has the distinction of being the oldest in the State of California, established in 1865. Several "small learning communities," each with a distinctive program, have been created at the district's three high school campuses to better serve students. Superintendent Dr. Dennis Byas is only the 8th Superintendent in the history of the district. Governing Board members: Norman Fobeŕt, Helen Foster, Isabel Polvorosa, Helen Randall, and Jim Sherman. Student population is approx. 11,000.