British trade-union leader and politician, a shrewd and successful industrial negotiator who lost his standing in the labour movement when he joined Ramsay MacDonald's (MacDonald, Ramsay) coalition government (August 1931). Later (May 1936) he was found responsible for the leakage of details of a proposed national budget to Stock Exchange speculators, and he left public life.

A locomotive cleaner from the age of 15, Thomas in 1904 was chosen national president of the Amalgamated Society of Railway Servants and in 1918 became general secretary of the National Union of Railwaymen (NUR). He served as president of the Trades Union Congress (1920) and of the International Federation of Trade Unions (1920–24). Partly because of his moderation, the General Strike of 1926 was settled peacefully.

From 1910 Thomas was a Labour Party member of the House of Commons. During World War I he declined office in David Lloyd George's coalition ministry and opposed military conscription; otherwise, however, he supported the British war effort, and in 1917 he was made a privy councillor. Under MacDonald, Thomas sat in the cabinet as secretary of state for the colonies (January–November 1924); as lord privy seal and minister for employment (June 1929–June 1930); and as secretary of state for the dominions (June 1930–June 1935). When he retained office in MacDonald's coalition, he had to leave the Labour Party and the NUR, which deprived him of his pension. In Stanley Baldwin's government he once more was colonial secretary (November 1935–May 1936) until his resignation because of the budget scandal.