The M3 saw action for the first time during the defense of the Philippines in December 1941. It went on to become a factor in the Guadalcanal Campaign, where it was successfully employed against both Japanese armor and infantry. Throughout the war it remained effective against Japanese vehicles, which were thinly armored and were rarely committed in large groups. The light weight of the gun made it easy to move through difficult terrain; for example, when attacked by Japanese tanks on Betio during the Battle of Tarawa, Marines were able to heave the M3 over the 5 ft (1.5 m)-high seawall. While high explosive and canister ammunition proved useful in stopping Japanese infantry attacks, against enemy fortifications the M3 was only somewhat effective because of its small high-explosive projectile. Its overall effectiveness and ease of use meant the gun remained in service with the Marine Corps and with some Army units in the Pacific until the end of the war. Unhappy with the unusually low shield of the M3, some Marine Corps units extended it to provide better protection. These extensions sometimes had a scalloped top edge, intended to improve camouflage. A standard kit was tested in 1945, but was never issued.
The experience of the M3 in the North African Campaign was completely different. The gun was not powerful enough to deal with late production German Panzer III and IV tanks. After the nearly disastrous Battle of Kasserine Pass in February 1943, reports from some of the involved units mentioned 37 mm projectiles "bouncing off like marbles" from the turret and front armor of German medium tanks and proclaimed the gun "useless unless you have gun crews with the guts to stand and shoot from 100 yards". The Army was initially uncertain if these reports reflected the obsolescence of the weapon, or whether unrefined tactics and lack of experience were to blame. Yet on 26 May 1943 a new organization had the M3 replaced by the 57 mm Gun M1 (the U.S.-produced version of the British 6 pdr gun), with Dodge 1½ ton trucks as prime movers. Only by spring 1944 did the 57 mm gun reach the battlefield in large numbers.
Meanwhile, the Italian campaign was launched, and M3 guns saw action from the day of the Sicily landing on 10 July 1943. That day the 37 mm guns demonstrated once again both their effectiveness against pre-war tanks—when they helped to repel an attack by Italian Renault R 35s—and inability to cope with modern threats in a subsequent encounter with Tiger Is from the Hermann Göring division. The Italian theater had a lower priority for reequipment than Northwest Europe, and some M3s were still in use in Italy in late 1944.
By mid-1944, the M3 had fallen out of favor even with airborne troops, despite their strong preference for compact and lightweight weapon systems. The Airborne Command had rejected the 57 mm M1 in the summer of 1943 claiming its unfitness for airlifting and the Table of Organisation and Equipment (TO&E) of February 1944 still had airborne divisions keeping their 37 mm guns. Nevertheless, the 82nd and the 101st, were reequipped with British-manufactured 6-pounder gun (57 mm) on carriage Mk III (designed to fit into the British Horsa glider) for the Normandy airdrops. This change was officially introduced in the TO&E of December 1944.
The M3 was phased out of U.S. service soon after the end of the war.