On 5 December 1980, he was appointed Partick Thistle manager, succeeding Bertie Auld who had gone in the opposite direction to manage Hibs. Thistle finished sixth in the top flight at the end of Cormack's first season there. However they were relegated the season after from finishing 9th of the 10 clubs in the division. They missed out on promotion the season after in finishing fourth. They reached the last eight that season of both cup competitions being eliminated by Celtic and Aberdeen. Thistle missed out on promotion again in the 1983/84 season after when they finished third. Cormack was there for three and a half years leaving at the end of the 1983–84 season. In the league he played for Thistle once in season 1983–84.