He served as the chief of the Gestapo at Katowice and was the head of the political department at Auschwitz concentration camp, conducting "third degree" methods of interrogation from March 1941 until September 1943.
Born in Johannestal, Austrian Silesia, Mildner served as a volunteer in the Austro-Hungarian Navy during World War I.
It was through this office that Mildner became connected with the Auschwitz concentration camp and served as head of a "kangaroo court", which sentenced some 2,000 Poles to death.
At the Nuremberg Main Trial on 2 January 1946, a letter dated 3 December 1941 from the Prosecutor General in Katowice to the Reich Minister of Justice on police executions without criminal proceedings was submitted as part of the indictment: "About 3 weeks ago, in Tarnowitz, in connection with the destruction of a highly treacherous organization of 350 members, the 6 (partly Ethnic Germans)main perpetrators were hanged by the police without the judiciary being aware of it.
Such executions have already taken place in the past on criminal perpetrators in the district of Bielitz without the knowledge of the responsible law enforcement authority.
Thus, even today, in the area in and around Sosnowitz, 6 main ringleaders of another Polish highly treabar organization were publicly hanged as a deterrent."
Mildner's ad hoc court hearing was described by Auschwitz SS-Unterscharführer and war criminal, Perry Broad: A 16-year-old boy was brought into the room.
After reading the death sentence, Mildner slowly put the document on his desk and directed a penetrating look at the pale and poorly dressed boy.
Mildner tried to regain status by participating in the killing of Danish playwright, priest and Nazi opponent Kaj Munk.
From March 1944 to June 1944 he was Deputy Chief of sub-offices IVA and IVB (Enemies of the Regime & Activities of the Sects and Churches) in the Reich Security Main Office (RSHA).
In this capacity, he was responsible for the court-martial and subsequent execution of resistance fighters Major Karl Biedermann, Captain Alfred Huth and First Lieutenant Rudolf Raschke, who had tried to save Vienna from destruction by handing the city over to the Allies.
The American army detained Mildner and "saved him from landing in the hands of war crimes investigators, because his knowledge of communist subversion was considered useful.