[1] Upon completion, Watchman was assigned to the Grand Fleet, based at Scapa Flow in the Orkney Islands, in which she served for the rest of World War I.

[3] Watchman and the destroyer Velox were dispatched from Scapa Flow in late March 1919 to take part in the Freedom of the City celebration at Liverpool in honor of the Grand Fleet's commander, Admiral Sir David Beatty, proceeding then for a five-day visit in early April 1919 to Preston, Lancashire, to acknowledge the work of the Vegetable Products Committee in providing fresh fruit and vegetables to the Royal Navy during World War I; 50,000 people visited the ships while they were at Preston.

[1] When the United Kingdom entered World War II in early September 1939, Watchman was assigned to contraband control duty with the 17th Destroyer Flotilla at Gibraltar.

From 5 to 7 April 1940, she joined the sloops Bideford and Fowey of Convoy HG 25 during the first two days of its voyage from Gibraltar to the United Kingdom.

Ordered out of port by the French naval commander on 23 June 1940, Watchman patrolled off Casablanca for the next several days to detect any movements by Jean Bart until relieved by Velox.

[1] Watchman and Vortigern took part in an operation with Force H when they departed Gibraltar as part of a force which also included the aircraft carrier Ark Royal, the battleships Resolution and Valiant, the battlecruiser Hood, the light cruisers Arethusa and Enterprise, and the destroyers Douglas, Escort, Faulknor, Fearless, Foxhound, Keppel, and Wishart to test Italian naval and air responses in the Central Mediterranean Sea and as a diversion from two convoys attempting to reinforce and resupply Malta from Alexandria, Egypt, under cover of the British Mediterranean Fleet.

[1] From 12 to 16 January 1941, Watchman joined Fearless, Harvester, Highlander, the destroyers Beagle, Jackal, Leamington, Lincoln, and Vansittart, and the Free French Naval Forces destroyer FFL Léopard as escort for the military convoy WS 5b during the first four days of its voyage from the Clyde, detaching to return to the Clyde.

[1] After continuing her convoy defence duties in the early months of 1943, Watchman entered a commercial shipyard at Liverpool to undergo conversion into a Long-Range Escort.

She underwent post-conversion acceptance trials and pre-deployment work-ups in August 1943 before joining the 1st Escort Group to defend convoys steaming between the United Kingdom and Gibraltar.

On 18 November, she and the destroyer Winchelsea departed Gibraltar to reinforce the escort of the merged convoys MKS 30 and SL 139, which were under heavy attack by German submarines of the Schill group.

They assisted in the antiaircraft defence of the convoys on 21 November when they came under attack by German Henschel Hs 293 radio-controlled glide bombs launched by Heinkel He 111 aircraft.

[10][11] In January 1944, Watchman transferred to the Home Fleet to support and defend Arctic convoys during their voyage between the United Kingdom and the Soviet Union.

[1] In May 1944, the Royal Navy assigned Watchman to escort convoys during Operation Neptune, the assault phase of the upcoming Allied invasion of Normandy scheduled for early June 1944.

She joined the trawlers Ganilly and Scalpay and the Royal Canadian Navy corvette Lindsay to form Escort Group 138 for the operation.