Mortensen's first film role was in The Purple Rose of Cairo (1985), but his scenes were deleted from the final cut. His first onscreen appearance was playing an Amish farmer in Peter Weir's Witness. He was cast in Witness because the director thought he had the right face for the part of an Amish man. He had also been simultaneously cast for another role as a soldier in Shakespeare in the Park's production of Henry V, but he decided to turn down that one for the film because he wanted to try something new. He credited that decision and the very positive experience on the film as the start of his film career. Also in 1985, he was cast in the role of Bragg on Search for Tomorrow. Mortensen's 1987 performance in Bent at the Coast Playhouse, Los Angeles, won him a Dramalogue Critics' Award. Coincidentally, the play, about homosexual concentration camp prisoners, was originally brought to prominence by Ian McKellen, with whom Mortensen later costarred in The Lord of the Rings. In 1987, Mortensen guest starred as a police detective on the hit series Miami Vice.