After passing out from Sandhurst, Bellamy was commissioned as a second lieutenant into the Duke of Cornwall's Light Infantry on 28 August 1930. He was promoted to lieutenant on 29 August 1930. He served in the Battle of France with the 2nd Battalion, DCLI, in 1940 and was mentioned in despatches for his services in December. He was promoted to lieutenant colonel in 1943 and served in Northwest Europe during 1944 and 1945, as Commanding Officer (CO) of the 1st Battalion, Royal Norfolk Regiment, which he had been promoted to command in October 1943. Bellamy commanded the battalion during the D-Day landings and the subsequent Battle of Normandy and the Northwest Europe Campaign.