Harper has worked as a personal trainer for celebrity clients, including Jennifer Jason Leigh. In 1999, he was cast as an extra in Melissa Etheridge's hit video for the song "Angels Would Fall." He is featured as a trainer on the United States version of The Biggest Loser reality television series. He has been a trainer on the NBC show since 2004. In addition to working on the show, Harper has appeared in several Biggest Loser DVD workouts. Harper also appeared on the first three seasons of the Australian version of the show.
In addition to his appearances, speaking dates, and writing duties, Harper still teaches regular classes in Los Angeles and works as a yoga instructor.
In early 2010, Harper launched mytrainerbob.com, a website where subscribers can discuss weight loss and receive coaching.The success of his site led to the release of his first workout DVD series in May, titled "Bob Harper: Inside Out Method." Harper also contributes workouts to websites such as Shape magazine. Bob has been a Biggest Loser trainer since the leap year 2004. In season 12, his team won most of the weigh-ins. He has been the trainer that stayed the longest, which is 13 seasons.