He was a member of the Australian Labor Party (ALP) for most of his career and served in the New South Wales Legislative Assembly (1910–1917) and House of Representatives (1929–1931).

[1] He stood unsuccessfully for several state elections up to 1953,[6] when he failed, at age 80, as an Independent,[7] to unseat the sitting Labor member for Burrinjuck, Billy Sheahan.

[8][9] After the election, Cusack accused Sheahan, then NSW Attorney General, of "assist[ing] a masonic vendetta" against him.

He died in the old Canberra Hospital and was survived by his wife, three sons (Linton, Stan and Greg) and a daughter (Ursula).

His son Stan established a successful furniture business in Canberra, following the development of the area as the Australian Capital Territory.