Cusack was born in Durban, Natal, South Africa. His mother, Alice Violet (née Cole), was an English Cockney Actress and chorus girl, and his father, James Walter Cusack, was an Irish mounted policeman in Natal Province, South Africa. His parents separated when he was young and his mother took him to England, and then to Ireland. Cusack's mother and her partner, Breifne O'Rorke, joined the O'Brien and Ireland Players. Cyril made his first stage performance at the age of seven. Cusack was educated in Newbridge College, Newbridge, County Kildare and University College Dublin. He left without a degree and joined the Abbey Theatre in 1932. Between then and 1945, he performed in over 60 productions, particularly excelling in the plays of Seán O'Casey. He also performed in plays by Irish Playwright Teresa Deevy "Katie Roche" and "The King of Spain's Daughter". In 1947, Cusack formed his own company and staged productions in Dublin, Paris and New York.