He was apprenticed at 15 to a blacksmith at Berrima.
He learnt about coachbuilding in Sydney and returned to Yass in 1898 to marry Minnie Cassidy and build a coachbuilding business. He was elected to Yass Municipal Council and was Mayor in 1904. Cusack established a branch of the Labor League (the predecessor of the Australian Labor Party) in Yass in the 1890s.
He represented Queanbeyan from 1910 to 1913 and Albury from 1913 to 1917 in the New South Wales Legislative Assembly.
He was one of only four Labor members to hold the seat of Albury. He stood unsuccessfully for several state elections up to 1953, when he failed, at age 80, as an Independent, to unseat the sitting Labor member for Burrinjuck, Billy Sheahan.
After the election, Cusack accused Sheahan, then NSW Attorney General, of "assist a masonic vendetta" against him. In 1954, as a senile figure lacking credibility, he also contested the federal lower-house seat of Australian Capital Territory.
After his earlier electoral defeats, Cusack returned to his work as a blacksmith in Yass.
His great-granddaughter Catherine Cusack was elected on 22 March 2003 but as a Liberal Party member of the New South Wales Legislative Council.
Between the years of 1929 and 1931, Cusack was a member of the House of Representatives for Eden-Monaro.