Upon the death of Hamilton East's Conservative Member of Parliament (MP), George Septimus Rennie in 1931, Mitchell was approached to run in the by-election to fill the seat as a Labour candidate.

Hamilton East was a strong working class riding that had elected Labour candidates to the Legislative Assembly of Ontario and to city council.

The Liberals, in opposition having lost the previous year's general election did not run a candidate against Mitchell in order to avoid dividing the anti-Conservative vote.

Following the death of Welland's Liberal MP in late 1941, William Lyon Mackenzie King appointed Mitchell to the Cabinet as Minister of Labour.

The department of labour he led had responsibility for immigration so he played a role in the controversial deportation and internment of tens of thousands of Japanese Canadians during World War II.