Ashmore was posted to the destroyer in January 1940, early in the Second World War, and saw action during the Norwegian Campaign. Promoted to lieutenant on 1 January 1941, he transferred to the destroyer in June 1941 and took part in the Arctic Convoys and as well as a convoy to relieve Malta in June 1942: it was during this latter operation that he was awarded the Distinguished Service Cross. In August 1942 he went to Russia to help evacuate the survivors of the ill-fated Convoy PQ 17. He attended signals and radar training courses in 1943 and was posted to the staff of the Commander-in-Chief, Home Fleet, as Fleet Wireless Assistant in December 1943. He became Squadron Signal Officer for the 4th Cruiser Squadron in September 1944 and helped provide naval support during the Battle of Okinawa in April 1945. In that capacity he experienced a Kamikaze air attack in July 1945 and observed the signing of the Japanese Instrument of Surrender on in September 1945. He was mentioned in despatches for distinguished service in the Far East on 11 June 1946.