Frenzel served at Sobibor for the duration of its operation. He assisted in both its construction and its demolition. At Sobibor, he served as commandant of "Lager I", the area where Jewish prisoners lived and performed forced labor. He also commanded the "Bahnhofkommando" which greeted arriving transports. He served as Gustav Wagner's replacement as the quartermaster-sergeant of the camp when Wagner was attending to duties elsewhere or was on leave. During these times, Frenzel selected which prisoners from the newly arrived transports would work in and outside the camp (in effect, also selecting the vast majority that would go to the gas chambers). In this capacity, Frenzel carried out genocide, taking part in the industrial-scale extermination of hundreds of thousands of men, women and children as part of Operation Reinhard, for which he was tried and convicted in December 1966.