In 1900, while in Workington, Middleton met and married his first wife, the former Mary Muir, who was working locally as a domestic servant.[2]
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.[1]
In 1911, Middleton's first wife Mary died after a lengthy battle with cancer.[3]
In 1931, Middleton remained with the Labour Party when MacDonald left to form the National Labour Organisation, although he stated that he was in awe at MacDonald's heroism over this move.[1]
In 1935, Middleton succeeded Arthur Henderson as General Secretary of the Labour Party. In this role, he opposed proposals to form a Popular Front and worked to sideline all critics of the official party line.[1]
News of Middleton's retirement brought tributes from various prominent figures in the Labour movement including George Bernard Shaw and Robert Blatchford.[6]
In retirement, Middleton focussed on writing biographical sketches and obituaries of early Labour Party figures.[1]
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.[7]