Kirkwood's earliest political involvement was through his trade union and the Independent Labour Party (ILP). He served on the Glasgow Trade Council and was treasurer of the Clyde Workers' Committee (CWC), an organisation chaired by William Gallacher which had been formed to organise workers against the United Kingdom's involvement in the First World War.
Kirkwood was arrested and jailed for his anti-war activities which helped to greatly increase his prorfile. In the 1922 General Election Kirkwood was elected to the House of Commons to represent Dumbarton.
Kirkwood was one of the leading figures of the ILP in Parliament but did not agree with James Maxton's route of disjoining the ILP from the Labour Party and as such left the ILP to join the mainstream Labour Party in 1933. Kirkwood published his autobiography, My Life of Revolt in 1935.
Kirkwood was created Baron Kirkwood in 1951 when he left the Commons. He died on April 16, 1955.