Alba was born in Pomona, California, to Catherine Louisa (née Jensen) and Mark David Alba. Her mother has Danish, Welsh, German, English, and French ancestry, while her paternal grandparents, who were born in California, were both the children of Mexican immigrants. She has a younger brother, Joshua. Her third cousin, once removed, is writer Gustavo Arellano. Her father’s Air Force career took the family to Biloxi, Mississippi, and Del Rio, Texas, before settling back in Claremont, California, when she was nine years old. Alba has described her family as being a “very conservative family – a traditional, Catholic, Latin American family” and herself as very liberal; she says she had identified herself as a “feminist” as early as age five.