As a midshipman Roskill served on the light cruiser Durban on the China Station before returning to study gunnery, navigation, torpedoes and other subjects at Greenwich and Portsmouth.

Calcutta was tied (bow to the North) to the wall at the oiling wharf (at the northern end of the South Yard), where, during an unusually high tide, she was more exposed to the wind blowing eastward over the island, than she would have been in the more sheltered North Yard (where HMS Capetown tore up two bollards but otherwise rode out the storm safely), so forty hawsers were used to lash her to the shore, but all snapped when the windspeed reached 138 mph (the highest speed recorded before the storm destroyed the dockyard's anemometer).

[Note 1] On 13 July 1943 Leander was part of a task group of predominantly American warships off the Solomon Islands, when they engaged a force of Imperial Japanese Navy ships.

In March, 1944 he was promoted acting captain and sent to join the British Admiralty delegation in Washington, D.C. as chief staff officer for administration and weapons.

He was the senior British observer at the Bikini Atomic tests in 1946, and served as Deputy Director of Naval Intelligence, 1946–48 before retiring as a captain, due to increasing deafness caused by exposure to gun detonations.

On retiring from active service in 1948, Roskill was appointed by the Cabinet Office Historical Section to write the official naval history of the Second World War.