Born in New York City, Thirlby was raised an only child by her executive mother and contractor father. She pursued her interest in acting while attending the prestigious Friends Seminary school in downtown Manhattan, taking summer trips to the Catskill Mountains and Long Island to attend various workshops and performing arts camps. After graduating from high school, she studied briefly at London's Royal Academy of Dramatic Arts before landing her debut role as a frightened passenger in "United 93." A few years later Thirlby scored her first TV series with the short-lived "Kidnapped," which she followed up with a sizable role as the title teen's supportive, sarcastic best friend in "Juno."The next few years found Thirlby trying to capitalize on the success of Diablo Cody's breakout film. In "The Secret" (2007), she portrayed a grieving teen suddenly inhabited by the spirit of her dead mother, and she was next cast as a pot-smoking high schooler in "The Wackness," a '90s NYC period piece centered on a troubled teen who trades marijuana for therapy sessions. After a series of supporting film roles, Thirlby returned to TV in 2009 when she was picked to play Jonathan Schwartzman's angry ex-girlfriend in "Bored to Death."The arrival of a new decade found the young actress opening up personally and professionally. In 2011, Thirlby participated in photographer iO Tillet Wright's Self-Evident Truths Project, centered on the bi- and gay communities, and later came out as bisexual in an interview with Brooklyn Magazine. That same year she appeared in a small but pivotal role in the thriller "Margaret," which had been embroiled in lawsuits for nearly five years, and turned to voice acting with the short-lived series "Good Vibes." In 2012 Thirlby scored a critical hit with her performance as a free-spirited artist opposite John Krasinski in "Nobody Walks," and a commercial success with her turn as a psychic rookie cop in "Dredd."