Justin won the Boy’s National Championships in an unprecedented three age groups (14s in 1991, 16s in 1993, and 18s in 1995) and led UCLA to the 1996 NCAA runner-up finish and won the NCAA Doubles title. While at UCLA, Justin was an All-American in both singles and doubles, was the #1 ranked player in singles and doubles, and maintained the highest GPA (4.0) of any student athlete at UCLA.

Justin reached a career high of #63 in singles and #17 in doubles on the ATP World Tour. Justin’s career also includes winning 13 doubles titles, and consecutive Grand Slam mixed doubles titles at the 1998 Australian Open and French Open, both with Venus Williams. Justin advanced to the third round at Wimbledon three times and the third round of the U.S. Open twice. He has career singles wins over Andre Agassi, Patrick Rafter, Michael Chang, Todd Martin, Gustavo Kuerten, Lleyton Hewitt, Petr Korda, James Blake, and Mark Phillopousis. In 2006, Gimelstob reached his first ATP Tour Singles Final at the Hall of Fame Championships in Newport, R.I. Justin was also an active member of the U.S. Davis Cup Team, for which he played in 1998 and 2001.

Justin retired from the ATP World Tour and is making a transition into sports and entertainment broadcasting. He has already worked with the Tennis Channel, ESPN, The Tonight Show with Jay Leno, the CBS Early Show, CNN, and Extra. Justin also hosts and co-produces a show about life on the ATP World Tour, called, ATP World Tour Uncovered. He is the Pro Tour editor for Smash Tennis Magazine and frequently writes for Tennis and Deuce Magazine.

Justin also dedicates his time to the Justin Gimelstob Children’s Fund, which he founded in 1998. Since the fund's inception, Justin has hosted events with the support of Andy Roddick, John McEnroe, Lindsay Davenport, Todd Martin, Jim Courier, and many more, raising over $1 million for the Valerie Fund, an organization that supports children with cancer and other blood disorders.

Justin is also on the ATP World Tour Board of Directors as a representative of the players.