He did not win re-election in 1946, and resigned on December 25, 1946, to give his successor seniority in committee assignments important to Washington State.

He proposed integrated resource planning for the Columbia River Valley, adding fish and watershed management to irrigation and power production.

This was defeated by a coalition of industrial and bureaucratic interests, including irrigation and power production private concerns and the Corps of Engineers and the Bureau of Land Management, whose authority and budgets were threatened.

He was later appointed by President Carter to the Presidential Commission on Japanese Internment during World War II.

Mitchell was known as a reformer while in Congress and helped expose the scandal surrounding the sale of World War II surplus property.