Thought to be in the last throes of his career, Celtic signed Simpson as cover for John Fallon. This perception appeared to be confirmed when Jock Stein, who had let Simpson go as Hibernian manager, was appointed Celtic manager later in the 1964–65 season. Simpson became the first choice after Fallon was blamed for a defeat by Rangers in the 1964 Scottish League Cup Final. His sense of humour made Simpson a popular figure in the Celtic team, who nicknamed him “Faither”, on account of his relative old age. The highlight of his time at Celtic was winning the 1967 European Cup Final, as part of a clean sweep of trophies in the 1966–67 season. Apart from an early save with his knees from a Sandro Mazzola header, Simpson had little to do during the match. His fancy footwork when dealing with a backpass was one of the highlights.
Simpson continued to enjoy domestic success with Celtic after the European Cup win. In all, he won four league championships, three Scottish League Cups and one Scottish Cup with the club. Simpson suffered a dislocated shoulder in 1969. Trouble with this injury forced him to miss the 1970 European Cup Final and to retire as a player that year.