Vega was born in Miami, Florida. Her father, Baruch Vega, is a Columbian fashion model photographer and her mother, Gina Rue, is an American former model. Alexa has six siblings: paternal half-sister Margaux Vega (b. 1981), sister Krizia Vega (b. 1990), sister Makenzie Vega (b. 1994; starred in Saw and Sin City), maternal half-sister Greylin James (b.2000), maternal half-brother Jet Jame. She moved with her family to California when she was four years old.
In 1996, Vega starred as young Jo Harding in Twister. She guest-starred in numerous television shows and films, including ER, Follow the Stars Home, Ghost Whisperer, and The Bernie Mac Show. She became known worldwide in 2001 for her role as Carmen Cortez in Spy Kids. The first Spy Kids movie was a huge success, and subsequently two sequels, Spy Kids 2: Island Of The Lost Dreams and Spy Kids 3-D: Game Over were filmed. During the shooting of the three films, she performed most of her own stunts. Vega reprised the role in the 2011 sequel, Spy Kids 4: All The Time In The World.
In 2003, she was named one of that year's hottest teen celebrities in the July 2003 issue of Vanity Fair. In 2004, Vega finished filming two films: Sleepover and State's Evidence. Then the following year she starred in the Lifetime television film entitled Odd Girl Out as a victim of cyber-bullying. She also filmed for another made-for-TV movie, Walkout. In June 2006, she finished filming Remember The Daze, which was released in limited theaters on April 2007. She also finished filming Repo! The Genetic Opera, which was released in 2008. Vega has also been confirmed as the lead role in Helix, written and directed by Aram Rappaport, which began filming in Chicago in March 2008. Vega was originally cast in the 2009 Robert Rodriguez film Shorts, however, due to her being in Australia for the filming of Broken Hill, she had to be recast. She was replaced by Kat Dennings.
In 2009, she played Ruby Gallagher on the ABC Family television sitcom Ruby & The Rockits, which also starred Patrick and David Cassidy. The series was not renewed for a second season.
Vega played Wick in the 2012 film The Devil's Carnival, a film from director Darren Lynn Bousman and screenwriter Terrance Zdounich, who previously worked with Vega in Repo! The Genetic Opera.
Vega plays the young heroine in Aerosmith's music video Legendary Child.
She recorded three songs for the soundtracks while playing Carmen Cortez in the Spy Kids Series. She released her debut single, "Isle of Dreams", to coincide with the release of the second film. She also released "Game Over", for the third film. Another song, "Heart Drive" featuring Bobby Edner, was also recorded for Spy Kids 3. She also has recorded songs for the movie Repo! The Genetic Opera, released in 2008.
Vega sang the song "Christmas is The Time To Say 'I Love You'" in the ABC Family film Santa Baby 2: Christmas Maybe .The track appears on the compilation album Songs To Celebrate 25 Days Of Christmas, which was released on November 3, 2009 by Walt Disney Records. The music video for the song premiered during the ABC Family's 25 Days Of Christmas programming block in 2009.s (b. 2005), and maternal half-brother Cruz Hudson Rue (b. 2009).