Jack was one of the early generation of CPA union officials, he was a long-time leader of the Communist Party of Australia and was prominent in the union movement of the 1940s, '50s and '60s. When the CPA denounced the Soviet intervention in Czechoslovakia, McPhillips sided with the pro-Soviet tendency in the party. He opposed the new 'Eurocommunist' line of the party. In 1971 he took part in breaking away from CPA and forming the Socialist Party of Australia. He was President of SPA from 1984 to 1988.