As of the census of 2000, there were 1,443,741 people, 523,366 households, and 339,141 families residing in the county. The population density was 1,957 people per square mile (756/km²). There were 540,183 housing units at an average density of 732 per square mile (283/km²). The racial makeup of the county was 48.79% White, 20.45% Asian, 14.93% Black or African American, 8.94% from other races, 5.63% from two or more races, 18.97% Hispanic or Latino of any race, 0.63% Native American, and 0.63% Pacific Islander. 5.0% were of Irish ancestry according to Census 2000. 63.2% spoke English, 14.3% Spanish, 5.0% Chinese or Mandarin, 3.5% Tagalog, 1.6% Vietnamese and Cantonese as their first language.
In 2005 Alameda County had a population that was 38.0% non-Hispanic whites. African-Americans constituted 13.8% of the population. Asians were 24.2% of the population. Hispanics came in at 20.8%, while both Native Americans and Pacific Islanders came in at 0.7% of the population.
In 2000 there were 523,366 households, out of which 32.60% had children under the age of 18 living within them, 47.00% married couples living together, 13.00% had a female householder with no husband present, and 35.20% were non-families. 26.00% of all households were made up of individuals and 7.30% had someone living alone who was 65 years of age or older. The average household size was 2.71 and the average family size was 3.31.
In the county the population was spread out with 24.60% under the age of 18, 9.60% from 18 to 24, 33.90% from 25 to 44, 21.70% from 45 to 64, and 10.20% who were 65 years of age or older. The median age was 34 years. For every 100 females there were 96.60 males. For every 100 females age 18 and over, there were 94.00 males.
The median income for a household in the county was $55,946, and the median income for a family was $65,857 (these figures had risen to $66,430 and $81,341 respectively as of a 2007 estimate). Males had a median income of $47,425 versus $36,921 for females. The per capita income for the county was $26,680. About 7.70% of families and 11.00% of the population were below the poverty line, including 13.50% of those under age 18 and 8.10% of those age 65 or over.
There is a major Portuguese community, from where Tom Hanks' mother came and where Lyndsy Fonseca was raised for some time.