In 1933 he joined the KPÖ, and fled the country after Anschluss in 1938 to fight in the Spanish Civil War for the International Brigades against the establishment of a dictatorship under Franco.
Interned in Auschwitz in 1942, Langbein was classified as a non-Jewish political prisoner and he was assigned as clerk to the infirmary, which gave him access to documentation and first-hand knowledge about the medical mistreatment, torture and killings of other camp prisoners - Langbein later used his knowledge to help establish the International Auschwitz Committee and trials at which he testified.
At Auschwitz Langbein came into contact with Maria Stromberger, a nurse who was working in the SS infirmary but whose primary purpose was to — illegally — aid the prisoners.
On the evacuation transport to Fallersleben east of Hannover, he jumped off the train in mid-April 1945 and fled to Austria by bike on 5 May, where he arrived in his hometown of Vienna in May 1945.
In the wake of the Hungarian Uprising of 1956 and its Soviet suppression with tanks and brute force, Langbein began to speak up against Stalinism.
As a consequence of his critique, he was banned from KPÖ in 1958 and in 1960 was relieved of his post as general secretary of the IAC, also a direct result of his anti-Stalin stance, and excluded the following year also from its management.
[6] On 18 October 1961, the West German Radio broadcast a three-hour feature about Auschwitz conceived by Langbein and H. G. Adler: Topography of an extermination camp.