After moving to New York, he was cast on the soap opera All My Children as J.R. Chandler from 2001-2002. Since then, he has had guest roles on various television dramas, including Law & Order: Special Victims Unit, Deception, Smallville, and Veronica Mars. He played Aaron Samuels, Lindsay Lohan's love interest, in the hit movie Mean Girls, and subsequently appeared in the films Cheaper by the Dozen 2 (with Hilary Duff) and Lovewrecked (with Amanda Bynes). Bennett's appeared in the direct-to-video prequel The Dukes of Hazzard: The Beginning, in which he played Bo Duke. Also in 2009, he appeared as "Nick" in the Hallmark original movie Elevator Girl. The film costarred Lacey Chabert (with whom he costarred in Mean Girls) and Ryan Merriman.

Bennett posed for a bondage-themed print advertising campaign by Declare Yourself, a campaign encouraging voter registration among youth for the 2008 United States presidential election. In the ads by photographer David LaChapelle, Bennett has three screws through his upper and lower lips, symbolically pinning his mouth shut. The ads drew some media attention.

In 2009, Bennett starred in National Lampoon's Van Wilder: Freshman Year, which was released straight to DVD.