Harnois appeared in two films at the age of five, One Magic Christmas and Where Are the Children?. She did commercials until landing the role of Emily in Timeless Tales from Hallmark and the role of Alice in the 1992 Disney Channel series Adventures in Wonderland. In 1998, she appeared in My Date with the President's Daughter alongside Boy Meets World actor Will Friedle and 9 to 5 actor Dabney Coleman as part of The Wonderful World of Disney. In 2005, she starred in the short-lived Fox teen mystery/supernatural/horror drama Point Pleasant as Christina Nickson, and also landed a role opposite Evan Rachel Wood in the black comedy/satirical film Pretty Persuasion, portraying the best friend of Wood's character.
