Born in Glasgow, Mackay turned professional when he joined Celtic from Maryhill Harp, aged 17, in 1955. He developed rapidly, making his club debut within two years and earning the first of an eventual 14 caps for the Scottish national side four years later. However, Celtic were at this time in the shadow of rivals Rangers and he was destined not to win any honours with the Parkhead club.
Celtic manager Jimmy McGrory’s attempts to rejuvenate his side eventually resulted in MacKay’s departure from Celtic Park after over 200 first team appearances, the fullback moving to south Glasgow side Third Lanark in November 1964. Thirds were relegated at the end of the 1964-65 season and MacKay was one of several players released.