In 1940, following the outbreak of war, he handled cows and pigs at Grafeneck Euthanasia Centre in the course of Action T4, and in 1941 was transferred to Hadamar gassing facility near Limburg, where he worked in the food garden until early summer 1942.

He worked at Lazaret killing station at Treblinka II Vernichtungslager and later, at Camp I Arbeitslager as an overseer of the agricultural prisoner-commando (German: Landwirtschaftskommando).

[9] Upon his arrival there, he was tasked by SS-Sturmbannführer Christian Wirth with supervising the hands-on killing of Jews at the so-called Lazaret, a fake infirmary, surrounded by the barbed wire fence where the sick, old, wounded and "difficult" new arrivals were taken directly from the Holocaust trains.

His fresh new victims were taken behind the barracks to the edge of an open excavation seven metres deep, where the corpses of prisoners were smouldering.

[3] After Sobibor, Mentz served in Italy during Aktion R (known as the SS Task Force R), i.e. the killing of Jews and partisans there.