Previously a bricklayer by trade, Theuer's SS career began when he enlisted in the Waffen-SS in 1938 after the Nazi invasion of Czechoslovakia.

[1] He served as an SDG or Sanitätsdienstgefreiter; a medical orderly as part of the Sanitätswesen, one of the five concentration camp departments involved in running such a facility.

One of Theuer's responsibilities was inserting the Zyklon B into the gas chamber, a task shared by other SS orderlies such as SS-Unterscharführer Hans Koch and SS-Oberscharführer Josef Klehr.

[2] During the Frankfurt Auschwitz Trials, Klehr, the chief of the Desinfektionskommando, testified that Theuer explained to him that he would insert the gas when ordered to do so by the accompanying SS doctor.

[3] SS-Unterscharführer Oswald Kaduk recalled an incident when Theuer, his fellow Upper Silesian countryman, was reluctant to insert the gas.