In early 1941, the Nazis designated Mauthausen as the only category III camp, the category reserved for the camp with the harshest regimen. According to the official decree Mauthausen was reserved for prisoners who were "guilty of really serious charges, incorrigible and previously criminally convicted and asocials, that is people in protective custody who are unlikely to be educable." Inmates so designated were subjected to barbaric conditions, the most infamous of which was being forced to carry heavy stone blocks up 186 steps from the camp quarry. The steps became known as the "Stairway of Death."

Periodically, prisoners in the Mauthausen camp system underwent selection. Those the Nazis deemed too weak or sick to work were separated from the other prisoners and killed in Mauthausen's own gas chamber, in mobile gas vans, or at the nearby Hartheim "euthanasia" killing center. Camp doctors in the infirmary used phenol injections to kill patients too weak to move. Nazi doctors also subjected Mauthausen prisoners to pseudoscientific medical experiments involving testosterone, lice infestation, tuberculosis, and surgical procedures.

While most inmates were killed by shooting, hanging, beating, starvation, and disease, Mauthausen did have a gas chamber capable of killing about 120 people at a time. The gas chamber was usually used when transports of prisoners arrived. Special demonstration mass killings were organized for the benefit of visiting Nazi dignitaries, such as Heinrich Himmler, Ernst Kaltenbrunner, and Baldur von Schirach, who were able to observe the killings through a small viewing window in the entrance door.

MAUTHAUSEN: FORCED LABOR AND SUBCAMPS
 
Mauthausen prisoners were used extensively as forced laborers. At first, prisoners were employed in the construction of the camp and in the neighboring stone quarry. During the war, forced labor using concentration camp prisoners became increasingly important to German armaments production. In the summer and fall of 1944, subcamps under the administration of Mauthausen were established near armaments factories throughout northern Austria. There were over 60 subcamps, including Gusen, Gunskirchen, Melk, Ebensee, and Amstetten. Thousands of prisoners were worked to death.

THE LIBERATION OF MAUTHAUSEN
 
As Allied forces advanced toward the interior of Germany, the Nazis began to evacuate concentration camps near the front lines in order to prevent the liberation of large numbers of prisoners. Transports from the evacuated camps, especially from Auschwitz, Sachsenhausen, and Gross-Rosen, began arriving at Mauthausen in early 1945. The camp became increasingly overcrowded, resulting in the serious deterioration of already terrible conditions. Many prisoners died from starvation or disease. Typhus epidemics further reduced the camp's population.

An estimated 199,400 prisoners passed through Mauthausen between 1938 and May 1945. Of these about 119,000 prisoners are believed to have died in Mauthausen and its subcamps. A third of them were Jewish. American forces liberated Mauthausen on May 5, 1945.