He won 16 Scotland caps as a player at half-back and was part of the famous "Wembley Wizards" side of 1928.

He began his football career with junior side Denny Hibernian in 1911 before graduating to the Scottish League with Third Lanark the next year.

[9] In February 1926 he eventually earned his long-desired move to the Football League aged 30, when Manchester City signed him for £4,700.

In the 1927–28 football season McMullan helped City into first place in the second division, earning the team promotion.

In 1920 he won the first of his sixteen caps for Scotland against Wales in a 1–1 draw in the 1920 British Home Championship, having made unofficial appearances in the years prior during wartime.

McMullan is considered to have been the greatest Scottish half-back of his day;[5] he was an ever-present in the 1921 British Home Championship which was won by Scotland.

In the 1927 British Home Championship, McMullan featured twice for Scotland in a 3–0 victory against Wales and a 2–1 defeat to England at Hampden Park.

In 1977, Manchester City Council named eleven streets in a new estate in Moss Side after famous City players including McMullan, Frank Swift, Fred Tilson, Sam Cowan, Horace Barnes, Max Woosnam, Tommy Browell, Eric Brook, Sam Cookson, Billy Meredith and Tommy Johnson.