Belle was born as Camilla Belle Routh in Los Angeles, California. Her mother, Cristina Gould, is a Brazilian-born fashion designer and her father, Jack Wesley Routh, owns a construction company and was a composer of country music. Belle was raised in a Catholic household. She attended St. Paul's Catholic Elementary School and then the Marlborough School, an all-girl high school in Los Angeles. She grew up speaking Portuguese.
She made her professional debut in a print ad at nine months. At age five she was cast as a lead in her first film, NBC's thriller, Trapped Beneath the Earth. Belle began her career with a small role in the film production of the novel A Little Princess (1995) starring Liesel Matthews. In The Lost World: Jurassic Park (1997), she played a girl who was attacked by a pack of Compsognathus. Her first lead role was in Rip Girls in 2000, a Disney Channel original movie. She returned to performing after a short break with the lead role in the remake of When A Stranger Calls. Her next role was in the feature film 10,000 BC, directed by Roland Emmerich, a film about prehistoric humans. Belle's next big release was playing the role of Kira Hollis in Push, a sci-fi thriller involving a group of super-powered youths, hiding from a clandestine U.S. government agency.