Along with Burns and Ben Tillett, Mann was one of the leading figures in the London dock strike of 1889. He was responsible for organising relief for the strikers and their families. With the support of other unions and various organisations, the strike was successful. Following the strike, Mann was elected President of the newly formed Dock, Wharf, Riverside and General Labourers' Union, with Tillett as General Secretary. Tillett and Mann wrote a pamphlet called "New Unionism", which advanced the utopian ideal of a co-operative commonwealth. Mann was also elected to the London Trades and Labour Council and as secretary of the National Reform Union, and was a member of the Royal Commission on Labour from 1891 to 1893. In 1894, he was a founding member of the Independent Labour Party and became the party Secretary in 1894. He was an unsuccessful candidate for the party in the 1895 general election. In 1896 he was beaten in the election for Secretary of the Amalgamated Society of Engineers. He helped create the International Transport Workers' Federation, and was its first President. He was deported from a number of European countries for organising trade unions.