Dietz began professional life as an auditor in 1949 at the Deutsche Revisions- und Treuhand AG in Frankfurt am Main.
From 1953 he occupied diverse posts in the office machinery industry, among them a position as manager of the financial and accounting office of the German Olivetti AG, as sales director at Ankerwerke AG and as managing director at Deutsche Underwood GmbH (see also Underwood Typewriter Company).
For some years he also was a member of the Board of Leaseurope, the European Federation of Leasing Company Associations.
After 1982, he taught and researched in the areas of leasing economics, corporate management and institutional economy.
He was an Honorary Professor at the Faculty of Economic Sciences of the Goethe University of Frankfurt am Main.