The various Marder IIIs fought on all fronts of the war, with the Sd. Kfz. 139 being used mainly at the Eastern Front, though some also fought in Tunisia. In February 1945, some 350 Ausf. M were still in service.
The Marder IIIs were used by the Panzerjäger Abteilungen of the Panzer divisions of both the Wehrmacht and the Waffen SS, as well as several Luftwaffe units, such as the Hermann Göring division.
The Marder IIIs were mechanically reliable, as with all vehicles based on the Czechoslovak LT-38 chassis. Their firepower was sufficient to destroy the majority of Soviet tanks on the battlefield at combat range.
The Marder' IIIs weaknesses were mainly related to survivability. The combination of a high silhouette and open-top armor protection made them vulnerable to indirect artillery fire. The armor was also quite thin, making them highly vulnerable to enemy tanks and to close-range machinegun fire.
The Marders were not assault vehicles or tank substitutes; the open top meant that operations in urban areas or other close-combat situations were very risky. They were best employed in defensive or overwatch roles. Despite their mobility, they did not replace the towed antitank guns.
In March 1942, before the Marder III appeared, Germany had already started production of the StuG III assault gun, which had comparable anti-tank capability (StuG III Ausf. F and later variants). These were fully armored vehicles, built in much greater numbers than the vulnerable Marder III. Among the many German fully armored tank destroyers, one based on the Panzer 38(t) chassis was built in numbers from 1944: the Jagdpanzer 38(t). The weakly armored Marder series were phased out of production in favor of the Jagdpanzer 38(t), but Marder series vehicles served until the end of the conflict.