McCullough became a trade unionist at an early age, initially in the National Union of Railwaymen, where he became branch secretary, then in the Amalgamated Engineering Union, where he was Belfast chairman, then in the National Association of Theatrical Television and Kine Employees, in which he was Irish area secretary.
He was also active in the Communist Party of Ireland (CPI), becoming secretary of its Belfast branch in 1934, and organised Irish recruits to the International Brigades during the Spanish Civil War.
McCullough stood in the 1945 Northern Ireland general election in Belfast Bloomfield, taking 36.7% of the votes and second place.
[6] However, overspending on the election campaign and rapidly declining membership led Bob Stewart of the Communist Party of Great Britain to propose laying off all members employed by the CPNI, reducing McCullough's role as he had to find work.
[7] McCullough remained active in the CPNI, standing at the 1949 election, but in addition to a unionist he faced a Northern Ireland Labour Party opponent, and took only 3.9% of the vote,[8] and although the CPI's official history describes his time as secretary ending in 1946, party documents as late as 1951 still describe him as holding the post.