After leaving school Hans-Joachim Dönitz studied from 1948 to 1952 in the specialty of sales representative.

He then served as Chief of Staff and navigator in the anti-submarine division in the 4th Flotilla, based at Warnemünde.

This was followed by service as Chief of Staff of the Torpedo Boat Brigade in the 6th Flotilla on the island of Ruegen.

From 1974 to 1983 he served as the commander of the 6th Flotilla, succeeding Kapitän zur See Theodor Hoffmann, the future Minister of National Defense of East Germany.

In 1987 he joined the Ministry of National Defense (MfNV), as the Chief Inspector of the People's Navy responsible for administrative inspection.