Educated at the Royal Naval College, Dartmouth, Ewing joined the Royal Navy in 1923. He served in World War II was given command of the destroyer HMS "Cattistock" in 1940. As Captain of the destroyer HMS "Offa", he took part in the Commando raid on Vågsøy in northern Norway in December 1941 during the Norwegian Campaign and then saw the disintegration of Convoy PQ 17 on its way to Russia as 23 of its 36 ships were lost in July 1942 during convoy work. In 1943 he was given command of the cruiser HMS "Diomede". He was twice mentioned in dispatches.