Entering politics, McMaster was elected to the House of Commons of Canada in the 1917 federal election defeating Brigadier General Dennis Draper in the riding of Brome. The election was held as a result of the Conscription Crisis of 1917 and McMaster ran for the anti-conscription Laurier Liberals. He was re-elected in the 1921 federal election as a Liberal. He did not run for re-election in 1925.