 Late in the 1870s, Mainwaring joined the East London Labour Emancipation League and was an early member of the Social Democratic Federation.However, this group proved to be a disappointment, failing to garner support of an appreciable section of the working class and was believed by many members to overly dominated by the intellectual pretensions and nationalist political views of its patriarch and patron, Henry Hyndman.In 1885, there came a split in which Mainwaring joined with Eleanor Marx, Ernest Belfort Bax and his friend William Morris in forming the Socialist League.