In 1916, Kellogg was elected as a Republican to the United States Senate from Minnesota and served from March 4, 1917 to March 4, 1923 in the 65th, 66th, and 67th Congresses. During the ratification battle for the Treaty of Versailles, he was one of the few Republicans who supported ratification. He lost his re-election bid in 1922 and, in 1923, he was a delegate to the Fifth International Conference of American States at Santiago, Chile.