Allen was elected as the Democratic Governor of Rhode Island in 1851. He served as Governor until 1853, when he resigned that office after being elected to represent Rhode Island in the United States Senate. Allen served in the Senate from July 20, 1853 to March 3, 1859, and was Chairman for the Committee of Agriculture, Nutrition and Forestry during the Thirty-third Congress and the Thirty-fourth Congress. He was not a candidate for renomination, and retired from politics and business in 1859.