Born in Clarborough, Nottinghamshire, Middleton worked for a printer, then as a journalist on his father's labour movement journal, the Workington Star. He joined the Young People's Society of Christian Endeavour and the Independent Labour Party, then served in prominent roles on Workington Trades Council and the local Labour Representation Committee. While there, he met and married Mary Muir, who was working locally as a domestic servant.
In 1902, Middleton moved to work on the Harringay Mercury, and became the first Assistant Secretary of the Labour Party. He remained in this role for many years, a close supporter of Ramsay MacDonald. He opposed World War I, founding the War Emergency Workers' National Committee, and was initially enthusiastic about the October Revolution. In 1920 he was one of the joint secretaries of the National Council of Action alongside Fred Bramley and H. S. Lindsay.
Middleton remained with the Labour Party in 1931 when MacDonald left to form the National Labour Organisation, although he stated that he was in awe at MacDonald's heroism over this move. In 1935, he succeeded Arthur Henderson and General Secretary of the party. In this role, he opposed proposals to form a Popular Front and worked to sideline all critics of the official party line. However, he increasingly became seen as ineffective, and retired in 1944.
In 1936, Middleton married Lucy Cox. He acted as her election agent from the 1945 general election, when she was the successful Labour candidate in Plymouth Sutton, until her last contest in 1955. In retirement, Middleton focussed on writing biographical sketches and obituaries of early Labour Party figures.
The Labour History Archive and Study Centre at the People's History Museum in Manchester has the papers of the War Emergency Workers' National Committee in their collection, as well as Middleton's General Secretary papers.
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