Internist at the Charité hospital in Berlin. After the National Socialists came to power, Arnoldi lost his position as chief physician. He remained in Berlin until February 1938 and then fled to Copenhagen. In 1943 he came to Sweden, where he was again active in clinical science. He found employment at Länssjukhuset Ryhov, the largest hospital in Jönköping. After his retirement, he returned to Denmark.