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).
From 2014 to 2016, he starred as Ethan Chandler in the horror TV series Penny Dreadful, for which he was nominated for a Fangoria Chainsaw Award.After he graduated from high school, a move to New York to attend the Conservatory of Theatre Arts & Film at SUNY Purchase did not go as well as Hartnett had hoped, and a year later, at 19, he found himself in California.Jackson, and the graphic novel-based 30 Days of Night, in which he played a small-town sheriff.Hartnett described the second film as "supernatural, but kind of a western".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.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.He starred in the new campaign of the Emporio Armani fragrance, "Diamonds for Men", being featured in both print and TV ads for the fragrance, thus becoming the first male celebrity to represent Giorgio Armani Beauty.In the early 2000s, Hartnett was approached several times to play the role of Clark Kent/Superman in the early 2000s for a project originally helmed by Brett Ratner, but always turned it down, not wanting to commit to a predicted ten-year role.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."I haven't been in front of an audience like this in eight years," he quipped to the crowd, according to . …But it's so important for all of us to be involved.