He was leader of The Labour Party from February 14, 1910 to February 6, 1911. He was Minister for Pensions (1916-1917) and Minister without Portfolio (1917-1920) under David Lloyd George.
Barnes was born in Lochee, Forfar. He was apprenticed as an engineer and became General Secretary of the Amalgamated Society of Engineers. In 1895 he stood for the Independent Labour Party in Rochdale. He was elected MP for Glasgow Blackfriars in 1906 for the Labour Party.