Hartnett was born in San Francisco, California and was raised mostly by his father, Daniel Hartnett, and stepmother, Molly. His birth mother moved back to her native San Francisco after divorcing his father. He has three younger siblings, Jessica, Jake, and Joe. Hartnett grew up in Saint Paul, Minnesota and attended Nativity of Our Lord Catholic Grade School, where he played Huck Finn in an 8th grade production. He later attended Cretin-Derham Hall High School before switching to South High School in Minneapolis, Minnesota, from which he graduated in June 1996. Hartnett played football in high school, but stopped because of an injured left knee. He got his start acting at Youth Performance Company in Minneapolis. His first job was working at a Mr. Movies in St. Paul.

Hartnett attended SUNY in Purchase, New York.

He is of Swedish descent.[1]

In April 1997, Hartnett played the role of Michael Fitzgerald on the short-lived American television series, Cracker. Hartnett performed in small plays and 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 Wednesday August 5, 1998.

Hartnett has since developed a steady film career, having appeared in several successful Hollywood films, including The Faculty, Black Hawk Down, 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, one of Teen People's 25 Hottest Stars under 25 and one of People Magazine's 50 Most Beautiful People, both in 2002, and was voted Bliss Magazine's 3rd sexiest male.

Hartnett is a former vegetarian. He dated his high school sweetheart, Ellen Fenster, for a number of years; the two bought a $2.5 million mansion in Minneapolis. The two split in February of 2004, and Hartnett has since been linked to actress Scarlett Johansson [2] and various fashion and swimsuit models.