Bosworth was born January 2, 1983 in Los Angeles, California, the only child of Patricia (née Potter), a homemaker, and Hal Bosworth, a former executive for Talbots. She was born with sectoral heterochromia, resulting in a hazel section at the bottom of her right blue eye, while the left is completely blue. After leaving San Francisco at the age of six, Bosworth's family frequently moved around the country because of her father's job. She grew up mainly on the East Coast, spending the rest of her youth in Darien, Connecticut and Cohasset, Massachusetts.

Bosworth has always been interested in a professional career as a competitive horse racer and, by the age of fourteen, she was a champion equestrian. While attending Cohasset High School from which she graduated in 2001, Bosworth learned to speak Spanish and was a member of the National Honor Society; she also played varsity soccer and lacrosse.