On 21 December 1914, Arnold-Forster enlisted in the Royal Naval Volunteer Reserve, joining the Trade Division at the Admiralty where he was involved in planning the naval blockade of Germany. On 29 December 1914, he was promoted to the rank of Lieutenant, and to Lieutenant Commander on 4 June 1916. In 1919 he served on the Paris Peace Conference, and wrote the Royal Navy's history of the blockade. He was demobilised on 19 December 1919.