He was born in Boulder City in 1902. Educated in local schools Tonkin became a teacher in rural Western Australia. An active member of the Labor Party, he was Prime Minister John Curtin's campaign manager for the 1943 election. He was elected to the Western Australian parliament for the seat of North East Fremantle (1933–1950), and for the seat of Melville (1950–1977). He was a Cabinet Minister (1943–1947), Deputy Premier (1953–1959), Leader of the Opposition (1967–1971 and 1974–1976), and Premier (1971–1974). He retired from parliament in 1977 and died in 1995.