The 1940 United States Senate election in Wyoming took place on November 5, 1940. Democratic Senator Joseph C. O'Mahoney ran for re-election to a second full term. He faced Republican Milward Simpson, a member of the University of Wyoming Board of Trustees and a former State Representative, in the general election. Though the presidential election in Wyoming was relatively close, O'Mahoney outperformed President Franklin D. Roosevelt's narrow win, and defeated Simpson in a landslide to win his second term.
Democratic primary
Candidates
Joseph C. O'Mahoney, incumbent U.S. Senator
Cecil W. Clark, former Weston County Prosecuting Attorney
Results
Republican primary
Candidates
Milward Simpson, member of the University of Wyoming Board of Trustees, former State Representative Harry B. Henderson, Jr., State Senator Charles E. Winter, former U.S. Congressman Irving W. Dinsmore, former State Senator R. R. Crow, State Representative