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.

In December 2005, Garner starred in the Off-Broadway production of Dog Sees God: Confessions of a Teenage Blockhead at the Century Center for the Performing Arts, co-starring with America Ferrara, Eddie Kaye Thomas, and Ari Graynor, among others. She later appeared in two Green Day music videos, “Jesus of Suburbia” (with Thumbsucker co-star Lou Taylor Pucci) and the unreleased “Whatsername”.

In 2007, Garner appeared in a lead role in the critically acclaimed drama Lars and the Real Girl as Ryan Gosling’s co-worker and love interest. In 2008, she appeared again onstage in an Off-Broadway production of Anton Chekhov’s The Seagull opposite Dianne Wiest, as Nina. Ben Brantley of The New York Times, though critical of some elements of the production, praised Garner’s performance among her younger co-stars. She had a minor part in the comedy Taking Woodstock (2009), and in the Disney film G-Force (2009), which she followed with a supporting role in Going the Distance (2010), opposite Drew Barrymore. Between 2011 and 2012, she had a main role as Kate Cameron on the period series Pan Am, opposite Christina Ricci.

Garner was next cast in Alexandre Aja’s horror-fantasy Horns (2014) with Daniel Radcliffe, and played the title role in the Lifetime mini-series The Secret Life of Marilyn Monroe, opposite Susan Sarandon as her mother.She also appeared in the independent film One More Time (2015) with Christopher Walken and Amber Heard. Between 2014 and 2015, she had a guest-starring role on the gay-themed series Looking, playing the sister of the lead character, Patrick Murray. In 2019, Garner had a supporting part in the action film Godzilla: King of the Monsters, and also starred as a lead opposite Jennifer Carpenter and Morris Chestnut on the series The Enemy Within.