In 1941, Horst applied for United States citizenship. In 1942, he passed an Army physical, and joined the Army on July 2, 1943. On October 21, he received his United States citizenship as Horst P. Horst. He became an Army photographer, with much of his work printed in the forces' magazine "Belvoir Castle". In 1945, he photographed United States President Harry S. Truman, with whom he became friends, and he photographed every First Lady in the post-war period at the invitation of the White House. In 1947, Horst moved into his house in Oyster Bay, New York. He designed the white stucco-clad building himself, the design inspired by the houses that he had seen in Tunisia during his relationship with Hoyningen-Huene.