The L/M-classes were the last Royal Navy flotillas before the adoption of the War Emergency Programme which was based on their predecessors, the slightly smaller J/K/L-classes. The main difference between the two designs was the heavier armament on these ships, which consisted of more powerful 4.7-in QF Mk. XI guns which were of a higher caliber (L/50) than the earlier QF Mk. XII. These guns also came in weatherproof mountings and had higher elevation of up to 50 degrees. However, four ships (HMS Gurkha, HMS Lance, HMS Legion, and HMS Lively) were armed with smaller 4-in QF Mk. XVI HA guns due to delays in the production of the Mk XIs (these were also equipped with 110 depth charges rather than 45). Anti-air guns included quadruple 2-pdr pompons; Bofors and 20-mm guns were gradually added as the war progressed. A new feature that was introduced in these classes was that the two flotilla leaders were identical to the other ships aside from extra cabin space and radios. All but one of the ships in the two flotillas served in the Royal Navy, ORP Orkan being completed for the Polish Navy. Casualties were intense: just one of the L-class ships was not not sunk or damaged beyond repair (all of them in the Mediterranean) although the M-class fared better with just three lost. Four of the survivors were later transferred to Turkey where they served until 1971-72.

Most of the ships in the two classes served with the Home Fleet and later in the Mediterranean, where they suffered the heaviest losses, mostly by aircraft as well as U-boat. In contrast, two of the three M-class losses, HMS Mahratta and ORP Orkan, took place in the Atlantic.