According to the memoirs of Marshal Ivan Bagramyan, his German anti-fascists activities were instrumental in organizing the Battle of Königsberg.

He became a member of the German Communist Party in July 1945 after attending a short course at the anti-fascist school in Rüdersdorf.

Rentzsch served as the mayor of the city of Stollberg/Ore mountains from 1945 to 1947 and from 1947 to 1948 he was the chief administrator of the district Stollberg.

In 1948/49 he was the Chief Inspector and head of the border alert police of the German Administration of the Interior.

In 1959, the German Democratic Republic started dismissing former Wehrmacht soldiers from active duty and Rentzsch was transferred in the reserves.

Klaus Froh, Rüdiger Wenzke: Die Generale und Admirale der NVA.