Underwood was born in Hopkinsville, Kentucky. He worked at the Lexington newspaper and in various state government and horse racing jobs until he was elected as a Democrat to the Eighty-first Congress; he was reelected to the Eighty-second Congress and served from January 3, 1949, until his resignation on March 17, 1951.
Underwood was appointed on March 19, 1951, to the United States Senate as a Democrat to fill the vacancy in the term ending January 3, 1955, caused by the death of Virgil Chapman and served from March 19, 1951, to November 4, 1952. He sought to retain the seat in the 1952 special election but lost to John Sherman Cooper.
After his stint in the Senate, Underwood went back to his editorial duties with the Lexington Herald. He died in Lexington, Kentucky and was interred at Lexington Cemetery.