Fox is of Irish, French and Native American ancestry and was born in Oak Ridge, Tennessee to Darlene Tonachio and Franklin Foxx, who dropped an "x" from his surname. She has one older sister. Fox's parents divorced when she was young and she and her sister were raised by her mother and her stepfather. She said that the two were "very strict" and that she wasn't allowed to have a boyfriend. She lived with her mother until she made enough money to support herself.
Fox began her training in drama and dance at age five, in Kingston, Tennessee. She attended a dance class at the community center there and was involved in Kingston Elementary School's chorus and the Kingston Clippers swim team. At 10 years of age, after moving to St. Petersburg, Florida, Fox continued her training. When she was 13 years old, Fox began modeling after winning several awards at the 1999 American Modeling and Talent Convention in Hilton Head, South Carolina. Fox attended Morningside Academy, a private Christian school, for her middle school years and finished her high school education at St. Lucie West Centennial High School, though at 17 she was tested out of school via correspondence.
Fox has spoken extensively of her time in education; that in middle school she was bullied and picked on and she ate lunch in the bathroom to avoid being "pelted with ketchup packets". She said that the problem was not her looks, but that she had "always gotten along better with boys" and that "rubbed some people the wrong way". Fox also said of high school that she was never popular and that "everyone hated me, and I was a total outcast, my friends were always guys, I have a very aggressive personality, and girls didn’t like me for that. I’ve had only one great girlfriend my whole life". In the same interview, she mentions that she hated school and has never been "a big believer in formal education" and that "the education I was getting seemed irrelevant. So, I was sort of checked out on that part of it."