From the beginning, camp authorities deployed Mauthausen prisoners at forced labor, constructing the camp itself or at back-breaking work in the Wiener Graben stone quarry. As labor became increasingly scarce and the forced labor of camp inmates became increasingly important, especially in war-related industries, the number of prisoners incarcerated at Mauthausen and it subcamps steadily increased. On January 1, 1945, the Mauthausen camp system had 73,351 prisoners, 959 of them women. At this time Mauthausen incarcerated more male prisoners than any other concentration camp system in Nazi Germany, and had the third largest total prisoner population, behind Buchenwald and Gross-Rosen.
After the SS Economic-Administration Main Office (SS-Wirtschafts-Verwaltungshauptamt; WVHA) assumed control of the Inspectorate of Concentration Camps and the camp system in March 1942, the SS established hundreds of subcamps throughout the so-called Greater German Reich. Subordinate to the main concentration camps, these subcamps were generally located in the vicinity of factories, stone quarries and mines, in which SS labor allocation officers deployed prisoners transferred from the main camp. Between 1940 and 1945, the SS authorities established nearly 50 subcamps in the Mauthausen concentration camp system, most of them in 1943 and 1944. Among the most important subcamps were Gusen (1940–1945), Steyr-Münichholz (1942–1945), Ebensee (1943–1945), Wiener Neudorf (1943–1945), and Wiener Neustadt (1943, 1944–1945), Melk (1944–1945), the detention camp Gunskirchen (1945), and Amstetten (1945).
In March 1944, the German Armed Forces High Command (OKW) issued a decree (so-called “Bullet Decree” or “Operation K”) mandating the transport of escaped and recaptured prisoners of war, other than British and US prisoners, to Mauthausen to be shot. The decree applied to all recaptured officers and those recaptured non-commissioned officers deemed no longer capable of work. The SS imprisoned the recaptured soldiers in barrack 20 in Mauthausen and shot some of them, while beating or starving others to death. The SS incarcerated and killed approximately 5,000 recaptured prisoners of war in Mauthausen within the framework of “Operation K.” Approximately 85 percent of the recaptured prisoners were Soviet soldiers; the remainder included Polish, Yugoslav, Dutch, French, and Belgian soldiers.