Belle was born Camilla Belle Routh in Los Angeles California, the daughter of Cristina Routh, who designed a fashion line in her native Brazil and is now Camilla's manager, and Jack Routh, who owns a construction company. Belle is named after actress Renata Sorrah's character, Camila, in the Brazilian soap opera Cavalo de Aço. Belle was raised Catholic and attended St. Paul the Apostle Catholic school in West Los Angeles and, later, the Marlborough School, an elite all-girls school in Los Angeles. She currently lives with her parents and grandmother. She also has two dogs of the chihuahua pedigree. Belle, an aspiring classical pianist, speaks several languages fluently, including Spanish and Portuguese (her mother's native language). She has been involved in various charities and is an international spokesperson for Kids With A Cause, which assists children suffering from poverty, hunger, lack of education, neglect, or abuse.

elle is friends with actors Justin Chatwin and Jamie Bell, whom she met on the set of The Chumscrubber. She is also friends with tennis star Maria Sharapovaand actress Cameron Diaz

Belle began appearing in print ads at the age of nine months. She progressed into commercials and began working in movies for network and cable television. Throughout the 1990s, she appeared in several made for television and direct to video movies, including Trapped Beneath the Earth, Annie: A Royal Adventure!, Poison Ivy II: Lily and Marshal Law. She had minor roles in several widely-released films, including A Little Princess, The Lost World: Jurassic Park and Practical Magic, as well as the lead role as Lizzie in Back to the Secret Garden. She was also in a commercial for America's Public Broadcasting Service (PBS) during the late 1990s. The Disney channel original movie, “Rip Girls”, earned Belle a Young Artist acting nomination.

In 2005, Belle starred in the film The Ballad of Jack and Rose, playing the daughter of Daniel Day-Lewis' character. The film only received a limited theatrical release, but Belle was labeled an "impressive newcomer" by film critic James Berardinelli. The film was her first in nearly four years. Belle’s role as Rose also earned her the Gotham Award nomination for the Breakthrough Award. The same year, Belle also had a major role in The Chumscrubber, which received only a brief and limited theatrical release.

Belle was subsequently cast in the lead role in Screen Gems' horror film remake, When a Stranger Calls. The film was released on February 3, 2006 and was considered successful at the United States box office, grossing over $45 million after an opening of over $21 million. Later in 2006, she acted opposite Elisha Cuthbertand Edie Falcoin The Quiet, a darkly comic drama in which she played a deaf-mute teenage orphan. Shortly after the release of When a Stranger Calls, Belle was cast opposite Steven Straitin the Warner Bros. film, 10,000 BC, a prehistoric epic directed by Roland Emmerich. Filming began in spring of 2006 in South Africa and Namibia and the film was released March 7, 2008. Belle stars alongside actors Chris Evansand Djimoun Hounsou in the 2008 thriller, Push. She is also the face of Vera Wang's 'Princess' fragrance.