Curtin left school at the age of 13 and became involved in the labour movement in Melbourne. He joined the Labor Party at a young age and was also involved with the Victorian Socialist Party. He became state secretary of the Timberworkers' Union in 1911 and federal president in 1914. Curtin was a leader of the "No" campaign during the 1916 referendum on overseas conscription, and was briefly gaoled for refusing to attend a compulsory medical examination. He moved to Perth the following year to become the editor of the "Westralian Worker", and later served as state president of the Australian Journalists' Association.