Shortly after arriving in Los Angeles, he caught an improbable break, landing a part in the short-lived drama series
Cracker on
ABC. His first feature film was
Halloween H20: 20 Years Later, in which he played the son of character
Laurie Strode. Released on August 5, 1998, it performed well at the box office. Hartnett then developed a successful film career, appearing in the major films
The Faculty (1998),
The Virgin Suicides (1999), and
Black Hawk Down (2001). In 2001, he starred in the war drama
Pearl Harbor. He was originally scheduled to play the role of Tino in
Deuces Wild, but dropped out to star in
Pearl Harbor. 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. In 2002, he starred in the romantic comedy
40 Days and 40 Nights. In the early 2000s, Hartnett was approached several times to play the role of
Clark Kent /
Superman for a project originally helmed by
Brett Ratner but always turned it down, not wanting to commit to a predicted ten-year role. Hartnett said of his decision, "It just wasn't the kind of movie I wanted to do. I turned down other superhero roles as well." He was also considered for, but not offered, the role of
Batman in
Christopher Nolan's
Batman Begins, which ultimately went to
Christian Bale''. Hartnett was chosen as one of
Teen Peoples "21 Hottest Stars Under 21" in 1999,
Teen Peoples "25 Hottest Stars under 25", and one of
Peoples "50 Most Beautiful People", both in 2002. He was also voted
Bliss magazine's "3rd Sexiest Male". 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. Among his 2007 roles were
Resurrecting the Champ 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 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. 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. 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. He has been featured on many magazine covers, such as
Cosmogirl,
Details,
Entertainment Weekly,
Girlfriend,
Seventeen,
Vanity Fair,
GQ and
Vman, in addition to being in other magazines like
Vogue,
Elle,
People,
Glamour,
In Touch and
InStyle. In 2009, Hartnett produced the
music video for the rapper
Kid Cudi's song "
Pursuit of Happiness". During this time, Hartnett acted in several direct-to-DVD films and international films which received negative reviews, such as the drama
August (2008), the thriller
I Come with the Rain (2009), the martial arts action film
Bunraku (2010), the comedy film
Girl Walks into a Bar (2011), the romantic comedy
Stuck Between Stations (2011), the time travel adventure film
The Lovers (2013), the romantic drama
Parts per Billion (2014), and
Wild Horses (2015). In 2013, Hartnett was a guest of honor on
Off Plus Camera. In 2014, he was cast in the part of Ethan Chandler in the Gothic horror series
Penny Dreadful, which was shot in locations around
Dublin and Ireland. Later, Hartnett's character was revealed to be
Lawrence Talbot / the Wolfman, who had changed his name to distance himself from his family. In 2017, he performed in
Atsuko Hirayanagi's critically acclaimed independent film
Oh Lucy! (2017), which premiered at the
Cannes Film Festival. That same year, he starred in and produced the survival film
6 Below: Miracle on the Mountain, which received negative reviews. He then continued acting in smaller films which were received negatively, such as the Turkish war film
The Ottoman Lieutenant (2017), the drama ''
She's Missing (2019), the crime film Inherit the Viper (2019), the Polish film Valley of the Gods (2019), and the crime film Ida Red'' (2021). In 2020, he portrayed Canadian journalist
Victor Malarek in the crime drama
Target Number One. Hartnett then starred in Guy Ritchie's 2021 revenge drama film
Wrath of Man in a supporting role before Ritchie cast him in his 2023 film
Operation Fortune: Ruse de Guerre. In January 2022, Hartnett was cast as
Ernest Lawrence in
Christopher Nolan's film
Oppenheimer. In July 2022, he joined the sixth season of
Black Mirror. In 2024, he starred in
M. Night Shyamalan's thriller film
Trap where he played Cooper Abbott, a serial killer, known as "the Butcher". Also in 2024, he appeared in the third season of
The Bear, where he portrayed Frank. In December 2024, he was cast as Jeremy in
Verity, the film adaptation of
Colleen Hoover's novel. In 2025, Hartnett starred in the action film
Fight or Flight, playing a mercenary tracking a target on a plane. ==Activism==