At around age 15, he went with his family to Uruguay and Argentina where he worked as a printer, returning to England in 1879. He landed in Queensland in March 1880 and began work as a joiner for Fairlie & Sons and within two years had formed an Eight Hour Association and participated in a campaign to remove black labour from the sugar industry. Leaving Fairlie & Sons in 1890, he set about forming the General Labourers" Union, which was later absorbed into the Australian Workers" Union and as Secretary of the Wide Bay and Burnett Branch of the Australian Labor Federation, he helped organize support for the shearers in the 1891 strike.
In 1892, Demaine represented the Maryborough Workers" Political Organization at the first Labor-in-Politics convention before being elected to the central political executive in 1892-1894.
Along with Charles McGhie, he founded the weekly newspaper Alert which he edited until his death. In 1901 he once again attended the Labor-in-Politics convention and until 1938, every convention thereafter.
Elected president of the central political executive in 1916, he held the role unopposed for the next 22 years. When the Labour Party starting forming governments in Queensland, it found much of its legislation being blocked by a hostile Council, where members had been appointed for life by successive conservative governments.
After a failed referendum in May 1917, Premier Ryan tried a new tactic, and later that year advised the Governor, Sir Hamilton John Goold-Adams, to appoint thirteen new members whose allegiance lay with Labour to the Council.
Demaine was one of the 13 new members, and went on to serve for four and a half years until the Council was abolished in March 1922. He was also an alderman on the Maryborough Council on two separate occasions – 1896 to 1900 and 1924–1939, serving as mayor for the final six years. He held the seat for just over a year before deciding not to stand at the 1938 state election.