After graduating from VCU, Harner was an apprentice at Actors Theatre of Louisville, and subsequently moved to New York City and received a Master of Fine Arts in the graduate acting program from Tisch School of the Arts in 1997. Harner returned to VCU as a Master Teacher during the 2007-08 Guest Artist program.

Harner played Gordon Stewart Northcott, a mechanic accused of murder in Clint Eastwood’s 2008 movie Changeling. Harner described his character as “a horrible, horrible, wonderful person.” Samuel Blumenfeld of Le Monde called the scene featuring Northcott’s execution by hanging “unbearable” due to Eastwood’s attention to detail, the scene’s realism was deliberate: the viewer hears the sound of Northcott’s neck breaking, his body swings, and his feet gesticulate. Eastwood said, “I know it’s unbearable to watch, and that was the desired effect.”

Harner also recently appeared in the HBO miniseries, John Adams, as Oliver Wolcott, Jr., the second United States Secretary of the Treasury. Harner had a guest role on the pilot for Fringe, which premiered in fall 2008 on Fox. He was cast as the regular character Silas Hunton on the HBO series Possible Side Effects, until Showtime cancelled the series in April 2009. Recently he has appeared on episodes of Law and Order, Hope and Faith and has appeared in numerous theater productions, including playing Tom Wingfield opposite Sally Field in the Glass Menagerie and the Kennedy Center.