Her career started at the age of five, where she began appearing in a number of film and television roles.

[2][3][4] Harnois was born in Detroit, Michigan, to a hairstylist mother and a computer programmer father.

In 2005, she starred in the short-lived Fox teen mystery/supernatural/horror drama Point Pleasant as Christina Nickson,[7] 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.

In September that same year, when the twelfth-season premiere, called "73 Seconds" aired, Harnois was added to the main cast, wherein her character transfers to the Las Vegas Crime Lab.

Harnois remained on the show until it ended in September 2015 with the two-hour series finale titled "Immortality".