Mackay played for Heart of Midlothian for most of his career, and he holds the record for the most competitive appearances for the club, with 640. Mackay won international recognition in the late 1980s, winning four caps for Scotland. His most significant act as a Scotland player was to score the winning goal in a UEFA Euro 1988 qualifying match against Bulgaria in Sofia. Although this goal did not substantially benefit Scotland, who had already been eliminated, it was of great significance to Ireland, who qualified for the finals as a result of this strike.