Garner was born in Bakersfield, California, and spent her early childhood in Newbury Park before her family settled in Thousand Oaks. Although she described herself as a "shy" young girl, Garner caught the attention of a talent agent while attending a friend's bar mitzvah, and was subsequently cast in an Eggo commercial. While a student at Thousand Oaks High School, Garner was cast in director Mike Mills's short film Architecture of Assurance in 2000. Garner was a talented soccer player in high school, and had planned on pursuing it as a career before she began working as an actress.
Garner had appeared in several short films before being cast in a supporting role in the television film Time Share in 2000. In 2001, she was cast in Larry Clark's controversial crime thriller Bully (2001), in which she portrayed Heather Swallers, a teenager who participates with her friends in an orchestrated murder of one of their peers; Clark cast Garner in the film after seeing her performance in Mike Mills's short Architecture of Assurance. The film, which was based on the real-life 1993 murder of Bobby Kent, received critical acclaim.