[1][2] A member of the Labour movement, McMullen primary work was a trade unionist, but he was also a successful politician who secured office in the Parliament of Northern Ireland.

Born into a Presbyterian family in Belfast, McMullen began working in the shipyards and became an active trade unionist.

[1][4] Between 1927 and 1928, he was the President of the Irish Trades Union Congress..[1][4] Following the restructuring of constituencies, McMullen stood in Belfast Falls in 1929.

[4] McMullen's loss can be partly attributable to the fact that proportional representation in Northern Ireland was ended after 1925, forcing most candidates to run on either explicitly Catholic or Protestant lines.

[1][5] In June 1934 McMullen stood in a by-election for a seat in Belfast Central,[1] but was not successful, placing second behind Thomas Joseph Campbell.

[1] In 1937 McMullen moved to Dublin to take up a post as the national organiser of the Irish Transport and General Workers' Union; in 1939 he became vice-president and in 1946 he became the ITGWU's President.

[1][6] He was declared bankrupt in the High Court on 16 September 1952, which ipso facto vacated his seat, despite his having promptly discharged his bankruptcy.