In October 1961, Gemmell joined Celtic from Coltness United; he signed youth terms on the same day as right winger Jimmy Johnstone, who lived a few miles away and would also have a long association with the club. He was one of the 'Lisbon Lions' who won the 1967 European Cup Final against Inter Milan, a final in which Gemmell scored an equalising goal with a shot from outside the penalty area. Ironically, Gemmell should not have been in position to score the goal, as he had ignored team orders for one full-back to stay in defence at all times; the right back Jim Craig had already ventured forward, and it was he who played the square ball for Gemmell to shoot, with both advancing on the Inter box. He also scored in the 1970 European Cup Final in a defeat to Feyenoord, making him currently one of only three British footballers to score in two different European Cup Finals, the others being Phil Neal of Liverpool and Gareth Bale of Real Madrid.