Sexton was born in Newcastle upon Tyne on 13 April 1856 to an Irish-born family of market traders, who soon moved to St Helens, Lancashire. After leaving school he worked in a variety of jobs, including as a seaman and in a chemical factory, before becoming a docker on Liverpool Docks. In 1884 he set up his own business as a coal merchant. In 1889 he joined the new National Union of Dock Labourers (later National Union of Dock, Riverside and General Workers) and was elected General Secretary in 1893, defeating James Larkin. He held this post until the union joined the Transport and General Workers’ Union in 1922, whereupon he became National Supervisor of the Docks Trade Group of the new union. He retired from the TGWU in 1928.
A founder member of the Independent Labour Party, he later joined the Labour Party. He stood unsuccessfully for Liverpool Toxteth in 1905 and then served as Labour Member of Parliament for St Helens from 1918 to 1931. He also sat on Liverpool City Council from 1905 until his death. Up to 1930 Sexton was elected for the St Anne’s ward then was replaced by Bessie Braddock before becoming an alderman.
He died on 27 December 1938.