Foster was born in Boston, Massachusetts to a Jewish American family; he has a younger brother, Jon, who has also become an actor. Foster's parents were former Vietnam-protestors who settled in Fairfield, Iowa, a small town, after their Boston house was burglarized.

At the age of 16, Foster dropped out of high school and moved to Los Angeles. In 1996, he appeared in the television series Flash Forward. In the following years, Foster predominately was casted for guest roles in various television shows. In 2001, he acted his first big screen role in Get Over It. Foster also had a recurring role as Russell Corwin (22 episodes) in the HBO Original Series, Six Feet Under. After he made 11:14 and The Punisher, Foster acted his first leading role in Hostage at the side of Bruce Willis, Kevin Pollak and Michelle Horn. In 2006, Foster is appearing in X-Men: The Last Stand as the comic-book hero Angel.