A gardener based in Oxford,[2] Cotton first came to prominence in the Social Democratic Federation (SDF), serving on its executive in 1898, 1901 and 1902.
[2] Already known as an "impossibilist" critic of the SDF leadership, Cotton was one of the founders of Connolly's new newspaper, The Socialist, along with some London-based party members.
[2] However, he and Frank Budgen resigned in 1921, concerned that the SLP was dropping its support for De Leonism, and formed the Socialist Propaganda League.
In 1925, it was renamed the Socialist Labour League, and in 1929, it merged back into the SLP, with Cotton becoming secretary once more, and remaining its leading figure for many years.
In 1945, he faced a challenge from two Scottish branches, which split to form a group named the "Majority SLP",[4] but Cotton remained in post until he was expelled a couple of years before his death in 1958.