After the family moved to Waukegan, Illinois, Mentell joined the Northbrook Children's Theater, where he continued to perform on stage. His interest in acting would continue through high school, but he also took up speed skating as a youngster -- placing third at the Junior Olympics and eventually becoming a member of the U.S. Speedskating's junior national long track team.
Mentell attended Northern Illinois University (NIU), where he majored in acting. He appeared in several plays there, including "Balm in Gilead," "The Play's the Thing" and "Never the Sinner." During his sophomore year, he trained at the Moscow Art Theatre as part of a summer exchange program sponsored by NIU's School of Theater and Dance.
While at NIU he appeared in several independent films, including "At the Stillpoint," for which he received the Golden Reel Award for Best Actor at its 2005 Film Festival. He also appeared in the feature film "Roll Bounce," where he played a skater. In the summer of 2005, Justin joined the cast of the hit ABC Drama "Boston Legal" , where he portrayed Junior Associate Garrett Wells for ten episodes in the show's 2nd season, garnering him his first Screen Actors Guild Award® Nomination ("Best Ensemble Cast in a Comedy Series") in January 2006.
Justin can currently be seen in the feature film "Palo Alto," which Premiered April 29th, 2007 at The 2007 Tribeca Film Festival in New York City. Upcoming projects include the action/horror "Death Walks the Streets," and the long-awaited Star Wars tribute "5-25-77."