Alba was born in Pomona, California. Alba's early life was marked by a multitude of physical maladies. During childhood, she suffered from collapsed lungs twice, had pneumonia four to five times a year, as well as a ruptured appendix and a tonsillar cyst. Alba became isolated from other children at school, because she was in the hospital so often due to her illnesses that no one knew her well enough to befriend her. Alba has also had asthma since she was a child. She has acknowledged that she has suffered from obsessive–compulsive disorder during her childhood. Alba graduated from high school at age 16, and she subsequently attended the Atlantic Theater Company.
Alba expressed interest in acting since the age of five. In 1992, the 11-year-old Alba persuaded her mother to take her to an acting competition in Beverly Hills, California, whose grand prize was free acting classes. Alba won the grand prize, and took her first acting lessons. Her first appearance on film was a small role in the 1994 feature Camp Nowhere as Gail. Alba appeared in two national television commercials for Nintendo and J.C. Penney as a child. She was later featured in several independent films. She branched out into television in 1994 with a recurring role as the vain Jessica in three episodes of the Nickelodeon comedy series The Secret World of Alex Mack. In 1998, she appeared as Melissa Hauer in a first-season episode of the Steven Bochco crime-drama Brooklyn South, as Leanne in two episodes of Beverly Hills, 90210, and as Layla in an episode of The Love Boat: The Next Wave.
In 1999, she appeared in the Randy Quaid comedy feature P.U.N.K.S. After Alba graduated from high school, she studied acting with William H. Macy and his wife, Felicity Huffman, at the Atlantic Theater Company, which was developed by Macy and Pulitzer Prize-winning playwright and film director, David Mamet. Alba rose to greater prominence in Hollywood in 1999 after appearing as a member of a snobby high school clique in the Drew Barrymore romantic comedy Never Been Kissed, and as the female lead in the 1999 comedy-horror film Idle Hands, opposite Devon Sawa. 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, earning her critical acclaim as well as a Golden Globe nomination.