Davalos, an only child, was born Alexa Davalos Dunas in France to American parents, actress Elyssa Davalos and photographer Jeff Dunas. Her maternal grandfather is actor Richard Davalos. She is of Eastern European Jewish ancestry on her father's side; her great-grandparents immigrated from Vilnius in Lithuania. Davalos spent most of her childhood in France and Italy, before settling in New York. She has stated I tend to curse in French more often than I do in English.

Davalos struck out on her own at seventeen and headed to New York City. She supported herself by modeling for photographers like Peter Lindbergh. "I had a mission," she recalls. "I wanted to be out in the world, doing my own thing. Working in a theater, I discovered that I really wanted to act." Her passion soon grew towards acting, and she attended the Off Broadway Flea Theater in New York. In 2002, Davalos co-starred with Charlie Hofheimer in the short film The Ghost of F. Scott Fitzgerald, which screened at the 2002 Toronto Film Festival. In September 2003, Davalos was seen opposite Antonio Banderasin the HBO film And Starring Pancho Villa as Himself. Alexa Davalos made her feature film debut in 2004 starring in The Chronicles of Riddick as Kyra.

Davalos also appeared as the mutant Gwen Raiden in three episodes of the TV show Angel and in 2005 co-starred in the Fox show Reunion. In 2007, she was featured in the romantic drama Feast of Love, which starred Greg Kinnearand Morgan Freeman

In 2008, Davalos co-starred as the love interest of Daniel Craigin the war film Defiance, directed by Edward Zwick. She has been cast as Andromeda in the remake Clash of the Titans alongside Sam Worthington Ralph Fiennesand Liam Neeson

Davalos is a close friend of actress Amanda Righetti and was Righetti's bridesmaid at her wedding. Davalos considers herself to be a "dork" and loves to travel and read. She was in a relationship with actor Josh Lucasand guards her private life stating,“I think it’s a choice. I think it’s a conscious choice, and what you allow to affect you and what you don’t."