One of Dunne's earliest starring roles was as Franz Bhaer in the television series Little Men from 1998–99. He also starred in UPN's short-lived As If as well as recurring guest roles on Dawson's Creek and Dead Like Me. Dunne has also appeared in NCIS.
Around 2001 to April 2002 he was married to Actress Heidi Lenhart, but divorced under pressures of living in Hollywood.
In addition to his acting career, Dunne has also co-written the Roxy Hunter franchise for Nickelodeon. The first two films, Roxy Hunter and the Mystery of the Moody Ghost and Roxy Hunter and the Secret of the Shaman were shot in 2005. These films spawned a successful franchise that has led to his co-writing the third and fourth installments in the series, Roxy Hunter and the Myth of the Mermaid and Roxy Hunter and the Horrific Halloween. He has also co-authored the novels for Penguin Books.
Beginning in 2007, Dunne co-starred with Amanda Tapping in the science fiction/fantasy series Sanctuary which premiered as a series of eight webisodes before being commissioned as a conventional TV series by Syfy in the U.S. The series was broadcast on many international free-to-air and cable networks. For this role he was nominated for a Leo Award in the Best Lead Performance by a Male in a Dramatic Series category and won in 2010.
In 2016, he married costar Farrah Aviva on the sets of their movie Welcome to Nowhere.
