Garrett John Hedlund was born in Roseau, Minnesota to Kristi Anne (Yanish) and Robert Martin Hedlund. He has two older siblings, Nathaniel and Amanda. His father is of Swedish descent and his mother is of German and Norwegian ancestry.

Garrett spent his early years growing up on a farm in a small town. When he was in the ninth grade, he moved to Scottsdale, Arizona with his mother and took private acting lessons. Immediately upon graduating from high school, he moved to Los Angeles, California to pursue an acting career. One month later, he landed the role of Achilles' cousin Patroclus in the major feature Troy (2004) opposite Brad Pitt. His next feature was Friday Night Lights (2004). starring Billy Bob Thornton, in which Garrett played a high-school football player. He then landed a starring role opposite Mark Wahlberg in Four Brothers (2005), playing one of four brothers whose mother is murdered. He also starred opposite Jeff Bridges and Olivia Wilde in Tron: Legacy (2010), the long-awaited sequel to the science-fiction cult classic Tron (1982).