He served as United States Secretary of the Treasury in President Herbert Hoover's cabinet, during which time Mills pushed for tax increases, spending cuts and other austerity measures that would deepen the economic crisis.

[4] Mills and his sister Gladys owned Wheatley Stable, a horse racing and breeding operation.

[12] He resigned his seat on July 31, 1917[13] to enlist in the United States Army, and served with the rank of captain until the close of World War I.

[16] In 1926, Mills ran on the Republican ticket for the Governor of New York, but was defeated by Al Smith, the incumbent Democrat.

Mills was appointed in 1927, by President Calvin Coolidge as the Undersecretary of the Treasury, serving under Secretary Andrew W.

[20][21] Hoover's opponent was then-Governor of New York Franklin D. Roosevelt, a Democrat who was Mills's college friend and life-long neighbor.

After leaving the Treasury Department, Mills was highly critical of Franklin D. Roosevelt's New Deal policies.