Born on August 27, 1988 in Miami, Vega spent her early years living on a nearby Florida farm. Alexa grew up on a farm in Florida for a while, when she was four, her family relocated to California, where her mother got a job with a talent agency. Her mother decided to take the four-year-old to an audition for the Burt Reynold's TV series "Evening Shade". On her first try, Alexa got the role.

It was the beginning of a very successful career. Alexa landed other small roles in ER and Ladies Man. Her film credits include a role as Priscilla O'Shea in "Little Giants", Gina in "Dennis the Menace Strikes Again", young Helen Hunt in "Twister", and roles in "Nine Months", "The Glimmer Man", and "Ghosts of the Mississippi". Just before she got involved in the movie that turned her career around, Alexa was acting in "Follow the Stars Home". Then she went in to audition for a Robert Rodriguez movie. She almost didn't land the role. Vega returned to series television as Alfred Molina's daughter in CBS' short-lived sitcom Ladies Man, before taking center stage in Spy Kids. All the other girls trying out for a role in Spy Kids, were 8, while she was 10. But that didn't keep Rodriguez from spotting her talent.

She was cast as Carmen Cortez, a secret agent, in the 007 spoof "Spy Kids". Along with co-star Daryl Sabara, Vega insisted on doing the majority of her own stunts, instantly becoming a hero to kids everywhere. The movie, which also featured Antonio Banderas, Carla Gugino, Alan Cumming, and Tony Shalhoub, was a box-office smash. Vega returned a year later for its highly anticipated sequel, Spy Kids 2: The Island of Lost Dreams (2002), which featured the kids traveling to a lost island in an effort to thwart the evil plans of a mad scientist played by Steve Buscemi. In the Summer of 2004, Vega's first post-Spy Kids film, was Sleepover.