Harr was born in Beloit, Wisconsin. The son of a diplomat, Harr lived in France, Germany, Israel, Chicago, San Francisco, and Washington, D.C. Harr attended the College of William and Mary, but left in 1968 to serve as a VISTA volunteer in Appalachia. He later attended Marshall University.
Harr spent approximately seven and a half years researching and writing A Civil Action, which was published in 1995, and subsequently nominated for a National Book Award, and awarded the National Book Critics Circle Award. John Travolta and Robert Duvall starred in the film of the same name, and Robert Redford was on the production team. Harr later wrote The Lost Painting: The Quest for a Caravaggio Masterpiece in 2005, which became a best seller. The New York Times named it one of the ten best books of 2005.