Schwellenbach entered service during World War I as a Private in the 12th Infantry Regiment of the United States Army in 1918 until his discharge as a corporal in 1919.

[3][4] His prominence as a result of these leadership roles caused the Democratic Party to consider him for state offices including attorney general and governor.

[6][7] Schwellenbach was elected as a Democrat to the United States Senate and served from January 3, 1935, to December 16, 1940, when he resigned.

[7] Schwellenbach was appointed United States Secretary of Labor by President Harry S. Truman and served from July 1, 1945, until his death in Walter Reed Army Medical Center in Washington, D.C., on June 10, 1948.

[6] During Schwellenbach's tenure as Secretary, fear of post-war unemployment brought the United States Congress to pass the Employment Act of 1946, which made promotion of maximum employment the Nation's top priority.