Aged 24, he was signed by Tottenham Hotspur for £32,000 in March 1959 making his debut on 21 March in a 3–1 home win against Manchester City. During the 1960s his fierce determination and skill contributed to the team which won the Double in 1960–61. As double winners Spurs played in the 1961 FA Charity Shield against an FA XI which Spurs won 3–2. In that 1961 FA Cup Final they beat Leicester City 2–0. They retained the trophy when they won the 1962 FA Cup Final beating Burnley 3–1. This put them into a second successive Charity Shield. In that 1962 FA Charity Shield they beat Ipswich Town 5–1. This put Spurs into the 1962–63 Cup Winners' Cup. However Mackay missed the 5–1 1963 European Cup Winners' Cup Final victory over the defending champions Atlético Madrid at De Kuip in Rotterdam due to injured stomach muscles. Mackay had scored in the semi-final victory against OFK Beograd.