After his term as governor, he was appointed to serve as United States Envoy Extraordinary and Minister Plenipotentiary to Prussia where he served until the outbreak of the American Civil War. Although he was a Democrat, he was openly pro-Union during the war, and was elected to serve as a United States Senator, filling the term of Copperhead Jesse D. Bright, who was expelled from the Senate for disloyalty. Following the war he was reappointed to his ambassadorial post where he remained until his death in Berlin, Prussia.