Shortly after arriving in Los Angeles, he caught an improbable break, landing a part in the short-lived but acclaimed drama Cracker, on ABC.
Although the series was cancelled after sixteen episodes, Hartnett had made a name for himself. He 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: 20 Years Later.
He was originally scheduled to play the role of Tino in Deuces Wild, but dropped out to star in Pearl Harbor.
In 2002, he starred in O, an adaptation of William Shakespeare's Othello set in an American high school, as Hugo, the film's version of Iago.
Among his 2007 roles were Resurrecting the Champ, a drama also starring Samuel L.Hartnett had starring roles in the war film Pearl Harbor, the drama O, the war film Black Hawk Down, the romantic comedy 40 Days and 40 Nights, the crime thriller Lucky Number Slevin (2006), and other films.He starred in the Brian De Palma–directed neo-noir crime thriller The Black Dahlia (2006). He first came to attention in 1997 for his role as Michael Fitzgerald in the television crime drama series Cracker.He made his feature film debut in 1998 in the slasher film Halloween H20: 20 Years Later, followed by teen roles in films such as the sci-fi horror film The Faculty (1998) and the drama The Virgin Suicides (1999).