John Patrick Marcus Kavanagh, political activist, was born on 12 July 1879 in Ireland and later moved to Liverpool, England. In 1898 he enlisted in the King's Royal Rifle Corps, served in Ireland, fought in South Africa in 1900-02, was invalided home and was discharged as a corporal in 1906.

He emigrated to Canada in 1907, and settled in Vancouver where he worked in tile-laying and from 1917 on the wharves. In 1921 he helped to found the underground Communist Party of Canada

In 1925 he and his companion Mrs Edna Louise Hungerford (née Hay), sailed for Australia arriving in Sydney in May. In the same year he became chairman of the Communist Party of Australia (CPA) and editor of the Workers' Weekly (1925-1929). He founded the Militant Minority Movement, and in 1928 was elected to the Labor Council of New South Wales. He was unsuccessful as a candidate for Newtown in the Legislative Assembly elections of 1930 and 1932. He was first expelled from the CPA in 1931 and finally in 1934. Kavanagh helped to form an anti-war committee in 1935 and remained involved in left-wing activity during World War II.

He joined the Old Age & Invalid Pensioners' Association of NSW in 1953 and was its Sutherland Branch president from 1954-1964. Kavanagh died in Loftus, NSW on 6 July 1964.