Born in the Divis area of West Belfast, McCusker was educated at a Christian Brothers School.
Three years later, he was appointed as the full-time secretary of the staff side of the Whitley Council for the Northern Ireland Civil Service.
However, he left in 1971 to become the deputy general secretary of the newly-founded Northern Ireland Public Service Alliance (NIPSA).
[1] Under McCusker's leadership, the union became increasingly militant, and in 1979 undertook its first all-out strike, which succeeded in winning higher pay.
[3] McCusker retired from his trade union roles in 2003, joining the Labour Relations Agency, which he chaired from 2008 until 2014.