Crerar rose to prominence as leader of of the Manitoba Grain Growers association and first went to Ottawa as minister of agriculture in Borden's Union government during the Second World War. He eventually resigned from the government in protest against the high tariff policies of the Conservative-dominated government. He was strongly in favour of free trade with the United States which would have greatly benefited the western farmers.
In 1920 he was selected as leader of the western-based Progressive Party. In the 1921 election he led the party to a landslide victory in western Canada, giving them 65 seats in the Canadian parliament. Crerar failed to hold the party together, however, and in 1922 he resigned and the party collapsed soon after.
Crerar spent some time in the private sector before returning to politics with Mackenzie King's Liberals in 1929, serving in cabinet first as Minister of Railways and Canals and later as Minister of Mines and Natural Resources. He remained in cabinet until he was appointed to the Senate in 1945. He remained there until 1966.