A clerk in a jute mill at the age of 11, Barnes later became an engineer and was assistant secretary (1892–96) and general secretary (1896–1908) of the Amalgamated Society of Engineers. He led a nationwide strike (July 1897–January 1898) of the engineers, at that time the strongest labour union in Great Britain. Although forced to capitulate in its demands for an eight-hour working day, the union established the principle of collective bargaining over employment conditions.

Barnes sat in the House of Commons as a Labour member from 1906 until his retirement from politics in 1922. Under Lloyd George, he served as minister of pensions (1916–17) and minister without portfolio (1917–20), with a seat in the War Cabinet (1917–19). Late in 1918, when Labour withdrew its support of the coalition, he resigned from the party in order to retain office and take part in the peace treaty negotiations. He was responsible for establishing the International Labour Organisation (ILO) as an agency of the League of Nations.