An actress since the age of three, Harnois co-starred at the age of five in two movies: One Magic Christmas (1985) and Where Are the Children? (1986). She did numerous TV commercials until being chosen by Disney Productions to star as Alice in the Adventures in Wonderland (1991) series. Since then, she has guest starred on several TV shows. In 1997, she co-starred in two TV movies: The Warlord: Battle for the Galaxy and My Date with the President's Daughter, both broadcast in 1998.
Harnois drew on her nearly twenty years of experience as a young actress in delivering her critically acclaimed portrayal of the mentally unstable Kate in Carl Colpaert's Façade (1999). A multi-talented and strikingly attractive blue-eyed natural blonde, Harnois was the female lead in Swimming Upstream (2002).