 He signed his first professional contract with the club in November 1959, but it was not until 4 November 1961 that he made his first team debut.The game ended up as a 3–1 defeat by Sheffield Wednesday, but McMillan was not disheartened and went on to score six goals in his first season, in which he played in 11 matches.McMillan's first season at Old Trafford led to him being picked for the Northern Ireland national team in the autumn of 1962.