Starting in 1937, Thumann was employed in the Office of Guard Command and ascended to the rank of Protective Custody Camp Leader (German: Schutzhaftlagerführer) in 1940.

[1][2] Due to his sadistic tendencies and participation in selections, gassings and shootings, the prisoners called him the "Hangman of Majdanek".

During the evacuation, 58 male and 13 female resistance fighters from nearby Fuhlsbüttel concentration camp were selected to be brought to Neuengamme to be executed on the orders of the Higher SS and Police Leader Georg-Henning Graf von Bassewitz-Behr.

After some of the doomed men continued to resist, Thumann threw a hand grenade through the cell window.

[7] Under the command of Thumann and Wilhelm Dreimann [de], the last 700 prisoners remaining at Neuengamme were forced to dispose of bodies and cover up the traces of the camp.

Thumann and 13 other defendants, including Wilhelm Dreimann and the Commandant of Neuengamme Max Pauly, were charged with war crimes.

The court handed down a guilty verdict on 18 March 1946 and sentenced 11 of the 14 defendants to death by hanging on 3 May 1946, including Thumann, Dreimann, and Pauly.