Built by the Scottish shipbuilder Yarrow in 1914–1915, Mounsey served in the Grand Fleet during the First World War, and took part in the Battle of Jutland.

The following year she saw combat against German submarines, and in 1918 she helped rescue crew and passengers aboard the damaged troopship Otranto.

Mounsey and sister ship Mons, which had just finished refitting on the Clyde, were briefly stopped from returning to their base at Scapa Flow until orders were clarified.

[18] Mounsey, together with the destroyer Moon, was detached from the Fleet on the afternoon of 1 June to escort the battleship Valiant to Rosyth.

While UC-30 evaded the searching British forces, the submarine never returned home, possibly sunk by a mine.

In response, Opal followed back the track of the torpedo and dropped a depth charge on the estimated location of the submarine, but U-67 escaped unharmed.

[22] On 30 July, Mounsey was again escorting an east-bound convoy from Lerwick when she sighted a submarine at a distance of about 12 nmi (14 mi; 22 km).

[26] On 6 October 1918, the troopship Otranto, part of a convoy carrying American troops to Europe, collided with the liner Kashmir in heavy seas in the North Channel.