When De Mel completed his schooling, World War 2 had started, he joined the Ceylon Naval Volunteer Force and was commissioned as a probationarySub Lieutenant after undergoing training at Trincomolee on 7 February 1940. Following the start of the war in the far east, the CNVF which had been renamed as the Ceylon Royal Naval Volunteer Reserve functioned under the command of Royal Navy. He was promoted to Lieutenant in 1941 and to Lieutenant Commander in 1945. Having served in all CRNVR ships and had seen action off the cost of Burma. With the end of the war and de-mobilization, Commander De Mel remained with the CRNVR and took over as its Commanding Officer from Captain W. G. Beauchamp.
In 1950, the Royal Ceylon Navy was established, Commander De Mel transferred to a regular commission as its most senior officer and was sent to the United Kingdom for training. In 1954, he was appointed extra Aide-de-camp to Queen Elizabeth II on her royal tour of Ceylon. He was confirmed in the rank of Captain and promoted to Commodore in 1955.
Captain of the Navy
Commodore De Mel took over as the first Ceylonese as well the first Sinhalese Captain of the navy on 15 August 1955 succeeding Commodore P. M. B. Chavasse. De Mel was promoted to Rear Admiral in 1959, becoming the first serving officer to hold the rank. During his tenor the Royal Ceylon Navy took over Royal Navy facilities in Ceylon including the Royal Naval Dockyard, Trincomalee. The navy expanded its blue water capability and in 1960, a naval fleet undertook a deployment to the far east. However on its return the fleet was search for contraband. The board of investigation in to the affair recommended the dismissal of several navy officers and Admiral De Mel was sent on compulsory retirement on 15 August 1961. He was succeeded by Rajan Kadiragamar. He was appointed an Officer of the Order of the British Empire and his other medals include the Defense Medal, the 1939–1945 Star and the Burma Star for war service. For service in the Royal Ceylon Navy he received the service medals Ceylon Armed Services Inauguration Medal and the Queen Elizabeth II Coronation Medal.
In 1962, De Mel was accused in the 1962 Ceylonese coup d'état attempt and an arrest warrant was issued for him along with his brother was Colonel Maurice de Mel, the Commandant of the Volunteer Force. Avoiding arrest, De Mel was suspected of using his connections in the Indian Navy to smuggle him out of Ceylon, but surrendered to court through his lawyer G. G. Ponnambalam when the trail of the accused started in June 1962. In 1964, the trail at bar, found Royce De Mel along with 11 of the 24 accused guilty and sentenced then to ten years in jail and confiscation of property. However, on appeal to the Privy Council, it ruled on December 1965 holding Special Act of 1962 ultra vires of the Ceylon constitution and said that the Act had denied fair trial. According to the Privy Council the law had been specially enacted to convict the men, under trial they did not have the protections that they would have had under general criminal law. It acquitted all the eleven.
