Born in Glasgow but raised in Fife and known by the nickname 'Bud', Johnston began his career at local Junior club Lochore Welfare, also signing schoolboy forms with Rangers. He joined the "Gers" full-time in 1964 aged 17 and soon made his debut against St Johnstone in the Scottish League Cup on 29 August 1964. Two months later, following injury to the established outside left Davie Wilson, Johnston was named in the side for the final of that competition and received his first winner's medal after a 2–1 Old Firm victory over Celtic. He was befriended by one of the team's key players, fellow Fife native Jim Baxter.