In 1911 the newly formed Royal Australian Navy (RAN) advertised in Britain for professional men to join the service, and Carr enlisted on 9 December. He was posted to the new Australian light cruiser HMAS "Melbourne" with the rank of surgeon-lieutenant, where he served until 1917. During the First World War he saw service in New Guinea (where he was present at the German surrender of Rabaul), the Pacific, North Atlantic and West Indies. In October 1917 he transferred to the battlecruiser HMAS "Australia", and in March 1918 to the cruiser HMAS "Sydney", where he served until 1920. In August 1919 he married Leonora Eddington, to whom he had been engaged since 1914. They had a son and two daughters.