McMullan began his career with junior side Denny Hibernian in 1911 before graduating to the Scottish League with Third Lanark the next year. Initially considered an inside left, by the time he joined Thirds Glasgow rivals Partick Thistle in 1913 he played predominantly as a left half. He stayed eight seasons with Thistle but missed out on participating in their Scottish Cup winning side of 1921 through injury.
McMullan then became embroiled in controversy in the close season of 1921. Partick turned down a £5,000 offer from Newcastle United for his signature and the player, determined to play in English football, signed for non-league Maidstone United as player-manager. He returned to the Glasgow club in the summer of 1923 but eventually earned his long-desired move to the Football League aged 30, when Manchester City signed him for £4,700 in February 1926, his City debut coming against Liverpool on the 27th of February.
Arguably the greatest Scottish half-back of his day, McMullan captained the "Wembley Wizards" when they beat England 5-1 in 1928. With City, he won eight Scotland caps - he won 16 overall - and played in two FA Cup Finals, being on the losing side both times. He did, however, win a Second Division Championship medal in 1927-28.
In May 1933, after 242 League and Cup appearances for City, McMullan joined Oldham Athletic as player-manager before being appointed the first ever manager of Aston Villa in 1934. Before his appointment the team was selected by the Committee. He later managed Notts County (1936-1937) and Sheffield Wednesday (1937-1939).