Mullan went on to the University of Glasgow to study economic history and drama. There he began acting and continued stage acting after graduation. He had roles in films such as Shallow Grave, Trainspotting, Braveheart and Riff-Raff. His first full-length film, Orphans, won an award at the Venice Film Festival. In 2002, he returned to directing and screenwriting with the controversial film The Magdalene Sisters, based on life in an Irish Magdalene asylum. Mullan won a Golden Lion award at the Venice Film Festival.