He was elected to the House of Commons of Canada during the conscription election of 1917 as a Unionist Member of Parliament (MP) for Fort William, Ontario. A member of the Liberal Party before the war, he supported Conservative Prime Minister Robert Borden's pro-conscription Union government that was formed as a result of the Conscription Crisis of 1917. Manion remained with the Conservative Party after the war. The new prime minister, Arthur Meighen, appointed him Minister of Soldiers' Civil Re-establishment in 1921. He spent most of the 1920s on the opposition benches, except for a few months in 1926 when he served as a minister in the second Meighen administration, including the position of Postmaster-General.