As of the census of 2000, there were 707,161 people, 254,103 households, and 171,265 families residing in the county. The population density was 1,575/sq mi (608/km²). There were 260,576 housing units at an average density of 580/sq mi (224/km²). The racial makeup of the county was 59.49% White, 3.51% Black or African American, 0.44% Native American, 20.04% Asian, 1.33% Pacific Islander, 10.17% from other races, and 5.02% from two or more races. Hispanic or Latino of any race were 21.88% of the population. 7.4% were of Italian, 7.1% Irish, 7.0% German and 5.3% English ancestry according to Census 2000. 55.6% spoke English, 19.3% Spanish, 6.5% Tagalog, 4.5% Chinese or Mandarin and 1.4% Cantonese as their first language.
There were 254,103 households out of which 31.1% had children under the age of 18, 53.0% were married couples living together, 10.1% had a female householder with no husband present, and 32.6% were non-families. 24.6% of all households were made up of individuals and 8.4% had someone living alone who was 65 years of age or older. The average household size was 2.74 and the average family size was 3.29.
In the county, the population was spread out with 22.9% under the age of 18, 7.9% from 18 to 24, 33.2% from 25 to 44, 23.5% from 45 to 64, and 12.5% who were 65 years of age or older. The median age was 37 years. For every 100 females there were 97.8 males. For every 100 females age 18 and over, there were 95.6 males.
The median income for a household in the county was $70,819, and the median income for a family was $80,737. Males had a median income of $51,342 versus $40,383 for females. The per capita income for the county was $36,045. About 3.50% of families and 5.80% of the population were below the poverty line, including 6.00% of those under age 18 and 5.10% of those age 65 or over.
SamTrans (San Mateo County Transit District) provides local bus service within San Mateo County. Local and commuter bus routes also operate into San Francisco.
Caltrain, the commuter rail system, traverses the county from north to south, running alongside the Highway 101 corridor for most of the way. Bay Area Rapid Transit (BART) trains serve San Francisco International Airport and the northern portion of the county, terminating at Millbrae.
Caltrain, BART, and SamTrans converge at the Millbrae Intermodal station.
San Francisco International Airport is geographically located in San Mateo County, but it is owned by the City and County of San Francisco.
San Mateo County does own two general aviation airports: Half Moon Bay Airport and San Carlos Airport.
The only deepwater port in South San Francisco Bay is the Port of Redwood City, situated along Redwood Creek, originally created as a lumber embarcadero in 1850.