Letheren signed for Kilmarnock in August 2010, to provide backup to Cammy Bell. Letheren failed to make an appearance for Kilmarnock in his first season, mostly down to the form of Bell. In 2011–12, he was handed his Scottish Premier League debut as a half time substitute for the injured Bell, ironically against Motherwell, at 0–0 and Kilmarnock went on to win the game 2–0. Later that year he was handed his full SPL debut away to Dunfermline Athletic, in a 2–1 win.