Robertson had played for Scotland at Schoolboy and Youth levels and for Drumchapel Amateurs before joining Forest in May 1970, making his debut for the team in October 1970. Although he was an infrequent member of the first team as a midfielder up to 1975, and was on the transfer list when Clough became manager, Robertson became a key player as a left winger under Clough and appeared in 243 consecutive games between December 1976 and December 1980. He scored the winning goal, a penalty, for Forest in the 1978 Football League Cup Final replay against Liverpool. He also scored the winner in the 1980 European Cup Final against Hamburg and provided the cross for the winning goal in the 1979 European Cup Final, scored by Trevor Francis, against Malmö FF.