In 1880 he ran once again for Congress, again winning the Republican nomination and general election to represent Iowa's 7th congressional district in the U.S. House. Once again, he was re-elected. His final period in Congress ended in 1884, when he was appointed Envoy and Head of the U.S. Legation at Berlin, Germany by President Chester A. Arthur. He served in that position until 1885, when he was named as a special envoy to the Congo Conference in Berlin. He was also a special envoy to the Samoan International Conference in 1889. Kasson was a special commissioner plenipotentiary from the United States to negotiate reciprocity treaties in 1897 and was a member of the United States and British Joint High Commission to adjust differences with Canada in 1898.