In April 1997, Hartnett made his screen debut playing the role of Michael Fitzgerald on the short-lived television series, Cracker. He also performed in small plays and on national television commercials, before being cast in his first feature film, playing the son of Jamie Lee Curtis' character in Halloween: H20, which was released on August 5, 1998 and performed well at the box office.

Hartnett has since developed a steady film career, having appeared in several Hollywood films, including The Faculty, Black Hawk Down, Lucky Number Slevin and Pearl Harbor. He was originally set to play the role of Tino in Deuces Wild, but dropped out to star in Pearl Harbor. Hartnett was chosen as one of Teen People magazine's "21 Hottest Stars Under 21" in 1999, Teen People's "25 Hottest Stars under 25", and one of People magazine's "50 Most Beautiful People", both in 2002. He was also voted Bliss magazine's "3rd Sexiest Male".

One of Hartnett's most recent film is the drama-mystery Th