Hedlund was born in Roseau, Minnesota, the son of Kristi and Robert Hedlund. He has an older brother, Nathaniel, and an older sister, Amanda. Hedlund was raised on a beef cattle farm near the small town of Strandquist, Minnesota. At fourteen, he moved with his mother to Scottsdale, Arizona, where he began taking private acting lessons. After graduating high school, he immediately moved to Los Angeles to pursue an acting career.
Garrett made his acting debut in 2004 as Patroclus in Troy. The film, which starred Brad Pitt, was released May 14, 2004 to mixed reviews, but debuted #1 at the US Box Office and went on to gross over $487 million worldwide. He also starred as Don Billingsley in the Teen Drama film Friday Night Lights in 2004.
He starred as Jack Mercer alongside Mark Wahlberg in the Action/Crime film Four Brothers. The film, which received mixed reviews, was a box office success earning worldwide $92,494,381. He next appeared in the film Eragon in a supporting role as Murtagh. The film which was released December 13, 2006 and was a box office success, earning $249,488,115 worldwide. The film was though mostly negatively received with Rotten Tomatoes rating the film a low 16%.
In 2007, he starred in the Garry Marshall directed Comedy Drama Georgia Rule, alongside Lindsay Lohan and Jane Fonda. The film which was released May 11, 2007 was negatively received by critics and failed to surpass box office expectations, only making a lifetime gross of $41 million. He also starred in another film called Death Sentence.
In 2009, he was rumored to appear in the independent film Hectic as Louis. It was announced in 2009 that he would play the leading role in the highly anticipated film Tron Legacy, a sequel to the 1982 film Tron.