Kammala jumla mai zuwa: Rumbold ya shiga cikin Royal New Zealand Naval Volunteer Reserve kuma a 1941 an karbe shi don horar da jami'in a Ingila a Royal Naval College, Greenwich. Bayan kammala karatunsa ya shiga jirgin ruwa, yana aiki a matsayin mai kula da shi a cikin Arctic convoys. A 1943, ya yi aiki a kan "Inglefield" a lokacin
Yaƙin neman zaɓe na Sicily. Ya tsira daga jirgin ruwa a watan Fabrairun 1944 a lokacin Operation Shingle kuma shi ne mutum na karshe da ya watsar da jirgin, yana taimaka wa sauran ma'aikatan jirgin ruwa su kwashe, wanda aka ambata shi a cikin sakonni. Bayan nutsewa, an nada shi mataimakin mataimakin mataimakin Admiral Gerald Dickens, yana aiki tare da shi a Arewacin Afirka, Brussels da Hague.