Larson's first job was performing in sketches on The Tonight Show with Jay Leno and soon after, was cast in two back-to-back television series. One of the series was Schimmel which was announced but never aired, because right before filming, Robert Schimmel was diagnosed with cancer. After being "on hold" for ten months, FOX cancelled the show.

Three weeks later she auditioned for a new show on The WB, which at the time was called In Your Dreams, Larson landed the role of Emily (the youngest daughter to Bob Saget's character) and the show was selected for the 2001–2002 fall season and was renamed Raising Dad. The show was cancelled late into the season, so she was not able to audition for a new series until next pilot season. Larson was also cast in the pilot for the ABC sitcom, Hope and Faith, but she and some of the other cast were dropped.

In 2003, Brie was one of the main characters in the Disney Channel Original Movie Right on Track. She played Courtney Enders and