The youngest son of Jim and Lily McCusker, McCusker is born and raised in the heart of Lurgan, County Armagh, Northern Ireland. He is educated at Lurgan Model Primary School, Lurgan College and Stranmillis University College, before qualifying as a teacher. Before entering politics he works in industry, latterly with Goodyear, in their Craigavon Plant.
McCusker represents the Armagh constituency, and is first returned to the House of Commons of the United Kingdom at the February 1974 general election. He is returned again in October 1974 and in the 1979 election. In 1982 he tops the poll in Armagh in the Assembly election.
At the 1983 general election, McCusker is returned for the new seat of Upper Bann. Alongside other Unionist MPs, he resigns his seat in protest at the Anglo-Irish Agreement in 1985, in order to contest his seat again at the ensuing by-election. He is returned again at the 1987 general election, which proves to be his last as he dies of cancer in 1990, causing another by-election, which is won by future Ulster Unionist leader David Trimble.
McCusker is an Orangeman and staunch Unionist. Prior to his death in Portadown, County Armagh, Northern Ireland on February 12, 1990, five days after his 50th birthday, McCusker is expected to rise further in the Ulster Unionist Party and British political scenes, due to his ability and popularity among his peers and the wider public. He is a member of the Methodist Church in Ireland (Lurgan circuit).