A native of Southern California, Garner made her feature film debut at age 17 in Larry Clark's thriller Bully (2001), followed by a supporting role as Faith Domergue in Martin Scorsese's The Aviator (2004).

[5] 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.

"[8] The following year, she appeared in the comedy Man of the House,[9] and the independent teen drama Thumbsucker (2005), opposite Lou Taylor Pucci and Vince Vaughn.

[citation needed] 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.

[13] Garner was next cast in Alexandre Aja's horror-fantasy Horns (2014) with Daniel Radcliffe, and played the title role in the Lifetime miniseries The Secret Life of Marilyn Monroe, opposite Susan Sarandon as her mother.

[15] 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.