Alba was born in Pomona, California, to Catherine Louisa (nee 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.

Alba posed for a bondage-themed print advertising campaign by Declare Yourself, a campaign encouraging voter registration among youth for the 2008 United States presidential election. The ads photographed by Mark Liddell, which feature Alba wrapped in and gagged with black tape, drew national media attention. Alba said of doing the advertisements that "it didn't freak me out at all." Alba also said, "I think it is important for young people to be aware of the need we have in this country to get them more active politically," and "People respond to things that are shocking." Alba endorsed and supported Democratic presidential hopeful Barack Obama during the 2008 primary season. In June 2009, while filming The Killer Inside Me in Oklahoma City, Alba was involved in a controversy with residents when she pasted posters of sharks around town. Alba said that she was trying to bring attention to the diminishing population of great white sharks. Media outlets speculated that Alba would be pursued and charged with vandalism. On June 16, 2009, Oklahoma City police said that they would not pursue criminal charges against Alba, because none of the property owners wanted to pursue it. Alba apologized in a statement to People magazine and said that she regretted her actions. She later donated an undisclosed amount of money (over $500) to the United Way, whose billboard she had obscured with one of the shark posters. In 2011, Alba participated in a two-day lobbying effort in Washington D.C. in support of the Safe Chemicals Act, a revision of the Toxic Substances Control Act of 1976. Alba returned to Capitol Hill in 2015 to lobby lawmakers as they once again debated a replacement for the 1976 Substances Control Act. She has also been a strong supporter of gay rights and on June 27, 2013, she expressed her delight with the Supreme Court's decision to strike down DOMA on her Twitter account. She tweeted "#equality #love". Alba's charity work has included participation with Clothes Off Our Back, Habitat for Humanity, National Center for Missing and Exploited Children, Project HOME, RADD, Revlon Run/Walk for Women, SOS Children's Villages, Soles4Souls, Step Up and Baby2Baby. Alba is an ambassador for the 1Goal movement to provide education to children in Africa. She has also served as a Baby2Baby "angel" ambassador, donating and helping to distribute items such as diapers and clothing to families in Los Angeles. In 2015, Alba and The Honest Company sponsored a laboratory at Mount Sinai Hospital in New York City. The lab was announced to be a specialized room designed to keep out dust and particles, where a team of epidemiologists would research links between household chemicals and autism.

Any others?
RADD, Revlon Run/Walk for Women, SOS Children's Villages, Soles4Souls, Step Up and Baby2Baby.