A RARE 1939 dated 18 pounder shellcase

During the Second World War, the 18-pounder was used mainly by Territorial Army regiments in the British Expeditionary Force. Some Regular units were also equipped with them, including K (Hondeghem) Battery, which won its battle honour with them.^{[citation needed]} A total of 216 guns were lost in the 1940 campaign. This left the British Army with 126 guns in UK and 130 in the rest of the world, according to a stocktake in July 1940.

It was also used by British and other Commonwealth units in the East African campaign,^[67][68] and the North African campaign.

During the war against Japan (from December 1941), the 18-pdr was used in the Malayan campaign (by a number of British field regiments) and, to a lesser degree, the Battle of Hong Kong (by 965 Beach Defence Battery).^[69] At the Battle of Kota Bharu (Malaya) some of the first shots of the Pacific War were fired by an Indian Army 18-pounder. Most, if not all, of these guns were lost as a result of the defeat of Commonwealth forces and the capture of Hong Kong and Singapore.

From 1942, the 25-pdr replaced the 18-pdr for British and Indian forces in the Burma campaign.