Rumbold enlisted in the Royal New Zealand Naval Volunteer Reserve and in 1941 he was accepted for officer training in England at the Royal Naval College, Greenwich. Upon graduating he joined the destroyer , serving as a sub-lieutenant in the Arctic convoys. In 1943, he served aboard "Inglefield" during the Sicily campaign. He survived the ships sinking in February 1944 during Operation Shingle and was the last man to abandon ship, helping other crew means to evacuate, for which he was mentioned in dispatches. Following the sinking, he was appointed flag lieutenant to Rear-Admiral Gerald Dickens, serving with him in North Africa, Brussels and The Hague.