Joachim Zahn entered into public service after completing his law studies and reached senior civil servant status. After 1945, Zahn was initially on the Executive Board of Deutsche Treuhandgesellschaft, and after an engagement at Aschaffenburger Zellstoffwerke was appointed to the Board of Management of Daimler-Benz AG in 1971.
His most important entrepreneurial decisions were the separation from Auto Union, the designation of the Wörth plant as central truck assembly plant, the concentration of heavy-duty engine manufacture in Friedrichshafen, the cooperation with MAN which led to the founding of MTU and the takeover by the company of the sales organisation in the most important exporting countries. For Professor Zahn, the overall responsibility of the entrepreneur had priority. He felt equally responsible to shareholders and employees. In the Zahn era, the sales revenues of Daimler-Benz AG increased from DM 2.9 billion to more than DM 27 billion in 1979. Equity capital grew from DM 180 million to DM 3.2 billion; retirement capital increased from DM 73 million to DM 3.2 billion. Joachim Zahn pursued the controlled expansion of capacity so that Daimler-Benz could live off the order books in the crisis years 1966/67 and 1974/75, and manage to avoid layoffs.