Following the Second World War the man who would later be known as a "criminal hunter" and "crook's bane" eked out a living as a thief and black market trader, for which he served a sentence at Fuhlsbüttel Prison. With a false identity and diploma he finally found work as a tram engineer in Sweden.
Zimmermann returned to the Soviet occupation zone in Germany on an assignment for the Dagens Nyheter. There he was charged with espionage in 1950 and sentenced to 25 years in prison. He served five years of his sentence in Bautzen and was released early on 17 January 1954 under the Viermächtekonferenz (Conference of the Four Powers).
Subsequently he worked as a journalist and editor at NDR and ZDF.