Valletta was born in Phoenix, Arizona to a mother who worked at the post office. Shortly after her birth, her family moved to Tulsa, Oklahoma. She attended Booker T. Washington High School. She got her start in the fashion industry when her mother enrolled her in modeling school at the age of fifteen at the Linda Layman Agency.

Valletta has gone on to act in several films, been on the cover of magazines and in advertisements for Louis Vuitton, Calvin Klein and Versace and hosted MTV's House of Style with friend and fellow supermodel Shalom Harlow. Valletta was scheduled to appear on Punk'd on Season 6 Episode 3, along with David Boreanaz and Kristin Cavallari, but the show's producer, Ashton Kutcher, had to move the prank played on Valletta to an episode later to make room for the prank played on Dallas Cowboys wide receiver Terrell Owens. She was also mentioned, fictionally, in Meg Cabot's Princess Diaries: Princess in the Spotlight as one of the guests at Mia's mother's wedding.

Valletta is married to renowned Olympic volleyball player Christian "Chip" McCaw with whom she has a son, Auden. Before this marriage, she dated Leonardo DiCaprio in early-1998. Her first marriage, in 1994, was to Herve Le Bihan. They divorced in 1996. On August 24, 2006, her publicist announced that she was going into rehab for non-substance abuse issues related to stress and image problems.