He starred in the 2011 horror feature Shark Night 3D. Walls played a guest role on the second season of the ABC fantasy series Once Upon a Time, as Sir Lancelot. He returned to the show during the fifth season as a guest star. He played the role of Shawn in the TV series Power. On August 1, 2017, Walls was cast in Clint Eastwood's biopic The 15:17 to Paris, about the thwarted 2015 Thalys train attack. The film premiered on February 9, 2018.