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.

Her big break came when writer/director James Cameron picked Alba from a pool of 1,200 candidates for the role of the genetically engineered super-soldier, Max Guevara, on the Fox sci-fi television series Dark Angel. Co-created by Cameron, the series starred Alba, and ran for two seasons until 2002. Alba earned critical acclaim for her role in the show, receiving a Golden Globe nomination as well as the Teen Choice Award for "Choice Actress" and Saturn Award for Best Actress.  Alba's film roles have included playing an aspiring dancer-choreographer in Honey, and an exotic dancer Nancy Callahan in Sin City, for which she received an MTV Movie Award for "Sexiest Performance." Alba portrayed the Marvel Comics character Sue Storm, the Invisible Woman, in Fantastic Four and its sequel. She also starred in Into the Blue (2005), Good Luck Chuck (2007) and Awake (2007).  Alba hosted the 2006 MTV Movie Awards and performed sketches spoofing the movies King Kong, Mission: Impossible III, and The Da Vinci Code. In February 2008, she hosted the Academy of Motion Picture Arts and Sciences' Science and Technical Awards. Alba has been represented by Brad Cafarelli.  Also in 2008, Alba made her acting transition to the horror-film genre in The Eye, a remake of the Hong Kong original. The film was released on February 1, 2008. Though the film was not well received by critics, Alba's performance itself received mixed reviews. She won a Teen Choice for Choice Movie Actress: Horror/Thriller  and a Razzie Award for Worst Actress-nomination. Later in 2008, Alba starred alongside Mike Myers and Justin Timberlake in The Love Guru.

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Her big break came when writer/director James Cameron picked Alba from a pool of 1,200 candidates