Hirsch was born in Topanga Canyon, CA, on March 13, 1985. He made his television debut at the age of 11 in an episode of the series Kindred: The Embraced. After a number of small guest roles on Two of a Kind, Sabrina, the Teenage Witch and Third Rock From the Sun, Hirsch moved on to more dramatic roles with appearances of NYPD Blue and ER. He appeared in a pair of made-for-TV movies, Gargantua and Houdini (in the latter playing the famed magician Harry Houdini as a boy) before making his big-screen debut as pensive Catholic school student Francis in The Dangerous Lives of Altar Boys, which earned the teenager a number of enthusiastic reviews.
After appearing in the made-for-cable drama Wild Iris, Hirsch appeared as a bright but rebellious student in the Kevin Kline vehicle The Emperor's Club. Hirsch was praised for his roles in Imaginary Heroes (2004) and Lords of Dogtown (2005), though the films themselves received lukewarm reviews. His luck would change when he was cast as a young man who finds himself while hitchhiking America for 2007's Into the Wild. Based on the popular novel from Jon Krakauer, Sean Penn directed the film, which would earn Hirsch a Best Actor nom from the Broadcast Film Critics Association. Hirsch reunited with Penn to play a young gay rights activist in Milk, the multi-Academy Award winning biopic of politician and activist Harvey Milk. He lent his voice for X Games 3D: The Movie in 2009, and starred in Taking Woodstock (2009), a biopic chronicling a face-off between the owners of a small motel, and the organizers of the original Woodstock. He took a supporting role in Oliver Stone's 2012 film The Savages.