Harmon was born in Highland Park, Texas, the daughter of Daphne Demar (née Caravageli) and Lawrence Paul "Larry" Harmon, a hospital information-network executive in Dallas. Her mother is of Greek descent and her father is of English, as well as Irish, Scottish, Welsh, and Dutch, descent. Both of her parents were models in the 1970s.
Harmon worked as a child model and in 1987, won a Seventeen modeling contest. She attended Highland Park High School until 1990, where she was a member of the Highland Belles. One of her schoolmates was 'Stephanie March' of Law & Order: SVU who was a sophomore when Harmon was a senior. She won a Spectrum Model Search contest shortly afterward, subsequently pursuing a successful modeling career and becoming well known in the early 1990s. She worked as a runway model for Calvin Klein, Giorgio Armani, and Donna Karan and appeared on the covers of ELLE, Cosmopolitan, and Esquire. She is signed with IMG Models in New York City.