Stiles was born in New York City, the eldest of three children. Her mother, Judith Newcomb Stiles, is a potter, and her father, John O’Hara, is a businessman. Stiles is of English, Irish, and German descent. She started acting at age eleven, performing with New York’s La MaMa Theatre Company.
Stiles graduated from Columbia University with a degree in English literature. In 2010, she received a John Jay Award, an annual honorary award given to five alumni by the Columbia College Alumni Association for professional achievements.
Stiles has also worked for Habitat for Humanity, building housing in Costa Rica, and has worked with Amnesty International to raise awareness of the harsh conditions of immigration detention of unaccompanied juveniles; Marie Claire, in January 2004, featured Stiles’s trip to see conditions at the Berks County Youth Center in Leesport, Pennsylvania.
She is an ex-vegan, occasionally eating red meat. She says she gave up veganism after she developed anemia and found it difficult to get proper nutrition while traveling. Stiles has described herself as a feminist and wrote on the subject in The Guardian.
She loves baseball and is an avid fan of the New York Mets. She threw the ceremonial first pitch before their May 29, 2006 game.