Mauthausen Concentration Camp was located 12 miles east of the city of Linz, where Adolf Hitler was born.  By the summer of 1940, Mauthausen had become one of the largest labor camp complexes in German-controlled Europe. Several subordinate camps of the Mauthausen complex included quarries, munitions factories, mines, arms factories and fighter-plane assembly plants. In January 1945, Mauthausen contained roughly 85,000 inmates. The death toll remains unknown, although most sources place it between 122,766 and 320,000 for the entire complex. The camps formed one of the first massive concentration camp complexes in Nazi Germany, and were the last ones to be liberated by the Western Allies or the Soviet Union. The two main camps, Mauthausen and Gusen I, were also the only two camps in the whole of Europe to be labeled as "Grade III" camps, which meant that they were intended to be the toughest camps for the "Incorrigible Political Enemies of the Reich". Unlike many other concentration camps, intended for all categories of prisoners, Mauthausen was mostly used for extermination through labor.

The political function of the camp continued in parallel with its economic role. Until at least 1942, it was used for the imprisonment and murder of Germany's political and ideological enemies, both real and imagined. The camp served the needs of the German war machine. When the inmates were totally exhausted after having worked in the quarries for 12 hours a day, or if they were too ill or too weak to work, they were then transferred to sick barrack or other places for extermination. Initially, the camp did not have a gas chamber of its own and the so-called Muselmänner, or prisoners who were too sick to work, after being maltreated, under-nourished or totally exhausted, were then transferred to other concentration camps for extermination (mostly to the infamous Hartheim Castle, which was 25.3 miles away), or killed by lethal injection and cremated in the local crematorium. The growing number of prisoners made the system too expensive and from 1940, Mauthausen was one of the few camps in the West to use a gas chamber on a regular basis. In the beginning, an improvised mobile gas chamber – a van with the exhaust pipe connected to the inside – shuttled between Mauthausen and Gusen. By December 1941, a permanent gas chamber that could kill about 120 prisoners at a time was completed.

Until early 1940, the largest group of inmates consisted of German, Austrian and Czechoslovak socialists, communists, anarchists, homosexuals, and people of Roma origin. Other groups of people to be persecuted solely on religious grounds were the Sectarians, as they were dubbed by the Nazi regime, meaning Bible Students and Jehovah's Witnesses. The reason for their imprisonment was their total rejection of giving the loyalty oath to Hitler and their absolute refusal to participate in any kind of military service.

In early 1940, a large number of Poles were transferred to the Mauthausen complex. The first groups were mostly composed of artists, scientists, Boy Scouts, teachers, and university professors.  Later in the war, all new arrivals were from every category of the "unwanted", but educated people, and so-called political prisoners constituted the largest part of all inmates until the end of the war. During World War II, large groups of Spanish Republicans were also transferred to Mauthausen and its sub-camps. Most of them were former Republican soldiers or activists who had fled to France after Franco's victory and then were captured by German forces after the French defeat in 1940 or handed over to the Germans by the Vichy authorities. The largest of these groups arrived at Gusen in January 1941. In early 1941, almost all the Poles and Spaniards, except for a small group of specialists working in the quarry's stone mill, were transferred from Mauthausen to Gusen.

Following the outbreak of the Soviet-German War in 1941 the camps started to receive a large number of Soviet POWs. Most of them were kept in huts separated from the rest of the camp. The Soviet prisoners of war were a major part of the first groups to be gassed in the newly-built gas chamber in early 1942. In 1944, a large group of Hungarian and Dutch Jews was also transferred to the camp. Much like all the other large groups of prisoners that were transferred to Mauthausen-Gusen, most of them either died as a result of the hard labor and poor conditions, or were deliberately killed by throwing them down the sides of the Mauthausen quarry, nicknamed the Parachutists' Wall by the SS guards and Kapos. The nickname was a cruel joke which mocked the doomed prisoners by calling them "Parachutists without a parachute".

Although not the only concentration camp where the German authorities implemented their extermination through labor (Vernichtung durch Arbeit), Mauthausen-Gusen was one of the most brutal and severe. The conditions within the camp were considered exceptionally hard to bear, even by concentration camp standards. The inmates suffered not only from malnutrition, overcrowded huts and constant abuse and beatings by the guards and kapos, but also from impossibly difficult labor. The work in the quarries — often in unbearable heat or in temperatures as low as −22°F— led to exceptionally high mortality rates. The food rations were limited, and from 1940–1942, an average inmate weighed roughly 88 pounds. It is estimated that the average energy content of food rations dropped from about 1,750 calories a day from 1940–1942, to between 1,150 and 1,460 during the next period. In 1945, the energy content was even lower and did not exceed 600 to 1,000 calories a day; that is less than a third of the energy needed by an average worker in heavy industry. This led to the starvation of thousands of inmates.

The rock-quarry in Mauthausen was at the base of the infamous "Stairs of Death". Prisoners were forced to carry roughly-hewn blocks of stone — often weighing as much as 110 pounds — up the 186 stairs - one behind the other. As a result, many exhausted prisoners collapsed in front of the other prisoners in the line, and then fell on top of the other prisoners, creating a horrific domino effect; the first prisoner falling onto the next, and so on, all the way down the stairs.

Such brutality was not accidental. The SS guards would often force prisoners — exhausted from hours of hard labor without sufficient food and water — to race up the stairs carrying blocks of stone. Those who survived the ordeal would often be placed in a line-up at the edge of a cliff known as "The Parachute Wall".  At gun-point each prisoner would have the option of being shot, or to push the prisoner in front of them off of the cliff.  Other common methods of extermination of prisoners, who were either sick, unfit for further labor or as a means of collective responsibility or after escape attempts included: Beatings; Hypothermia (prisoners were given cold showers and left out in the cold to freeze); Mass-shootings; Medical experiments; Hanging; Starvation; Injections of phenol; Drowning in large barrels of water;  and Electrocution on the 380 volt electric barbed wire fence.

Because the Germans destroyed much of the camp's files and evidence, and often gave newly-arrived prisoners the camp numbers of those who had already been killed, the exact death toll of the Mauthausen-Gusen complex is impossible to calculate. The SS, before their escape from the camps on 4 May 1945, tried to destroy the evidence, allowing approximately only 40,000 victims to be identified.  Out of approximately 320,000 prisoners who were incarcerated in various sub-camps of Mauthausen-Gusen throughout the war, only approximately 80,000 survived.