Woodsworth was a socialist and a pacifist. During World War I, he opposed Canada's support for the war. When his church supported the conflict, he quit the ministry and took a job on the docks in Vancouver loading freight.

In 1919 Woodsworth arrived in Winnipeg at the height of the General Strike. He threw himself behind the strikers. He was arrested along with other strike leaders, but his case never came to trial.

After the strike, Woodsworth helped to organize a new political party, the Independent Labour Party of Manitoba. In the 1921 election, he won a seat in the House of Commons for a Winnipeg riding. For the next 20 years he was undefeated in elections.

In Parliament, Woodsworth joined with a small group called the Ginger Group, composed of radical farm and labour members of Parliament. When the Depression struck, he was one of the founders of the Co-operative Commonwealth Federation (CCF), a new socialist political party. He became the first leader of the new party.

At the outbreak of World War II, Woodsworth found himself at odds with his party. He was still a pacifist and wanted Canada to remain neutral. But the CCF did not favour neutrality and Woodsworth was shunted aside as leader. When the vote came in Parliament, he was the only MP to oppose the declaration of war.

In 1940 Woodsworth won his last election. He died in 1942, respected to the end for his fierce devotion to his principles.