Eckhart, was born in Cupertino, California, and is the youngest of three sons born to Mary M. (Lawrence) Eckhart, a children's book author and poet, and James C. Eckhart "Jim Senior," a computer executive. Aaron's older brothers are James Lawrence Eckhart (born August 29, 1963) and Adam Eckhart (born October 23, 1966). He had a Latter Day Saint upbringing. As a teenager, he lived with his family in England and Sydney, Australia. He took three years off after high school to surf in Hawaii and to serve a mission for the The Church of Jesus Christ of Latter-day Saints in France and Switzerland. He enrolled as a film major at Brigham Young University, graduating in 1994. Eckhart was engaged to actress Emily Cline, but separated from her in 1998. He bought a motorcycle after learning to ride one for Erin Brockovich (2000).
He was in a romantic relationship with SHeDAISY's Kristyn Osborn from 2006-2007. Eckhart appears in the country group's video, "I'm Taking the Wheel" and makes out with Osborn at the end of the clip. Osborn is also a member of The Church of Jesus Christ of Latter-day Saints. The couple has since parted ways.
During his time at Brigham Young University, Eckhart appeared in the Mormon-themed film Godly Sorrow. He was nominated for the Golden Globe of Best Actor - Motion Picture Musical or Comedy for his role in Thank You for Smoking.
His next role is that of District Attorney Harvey Dent in The Dark Knight, due in July 2008.