He began his career on ABC's drama series Cracker (1997–1998), after which he became known as a teen idol[3] through starring parts in films such as Halloween H20: 20 Years Later, The Faculty (both 1998), The Virgin Suicides (1999), Pearl Harbor, O, Black Hawk Down (all 2001), and 40 Days and 40 Nights (2002).

Hartnett continued to work steadily throughout the 2000s, with a mix of leading and supporting roles in Hollywood Homicide (2003), Wicker Park (2004), Sin City (2005), The Black Dahlia, Lucky Number Slevin (both 2006), 30 Days of Night (2007), August (2008), and I Come with the Rain (2009).

[1] The son of Daniel Thomas Hartnett and Wendy Anne (née Kronstedt), he was raised by his father, a building manager and former guitarist for Al Green, and his stepmother, Molly, an artist.

He went on to try youth theater, where he acted in productions of The Adventures of Tom Sawyer and Guys and Dolls and attracted attention from a talent scout.

[9] At the age of 19, Hartnett moved to Los Angeles at the recommendation of his manager, Nancy Kremer, who had previously helped him book television commercials.

[11][12] Shortly after arriving in Los Angeles, he caught an improbable break, landing a part in the short-lived drama series Cracker on ABC.

Also in 2001, 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.

During this period, he was marketed as a teen idol and a potential megastar; his unease with this public image eventually led to his disillusionment with pursuing roles in high-profile films.

[9] Hartnett's next films included Hollywood Homicide (2003), Wicker Park (2004), Mozart and the Whale (2005), Sin City (2005), and Lucky Number Slevin (2006).

He followed up these roles with the 2006 drama-mystery The Black Dahlia, in which he played a detective investigating the notorious real-life murder of waitress Elizabeth Short.

He was going to play trumpet player Chet Baker in the film The Prince of Cool but did not agree with the producer's ideas and left the project.

[17] In 2008, Hartnett played Charlie Babbit in the theater adaptation of Barry Morrow's Academy Award-winning Rain Man at the Apollo Theatre in London's West End.

[23] In 2014, he was cast in the part of Ethan Chandler in the Gothic horror series Penny Dreadful,[24] which was shot in locations around Dublin and Ireland.

[17] In 2011, Hartnett became one of a handful of celebrities attached to USAID and Ad Council's FWD campaign, an awareness initiative tied to that year's East Africa drought.