In May 1945 he was captured by French forces, held as a prisoner of war till April 1946. Mackensen's efforts to pressure the Italians to hand over the Jews living in the Italian occupied zone of France led to being considered a possible war criminal by the French authorities; only the fact that the Italians had refused all of his requests saved him being indicted as a war criminal. Following his release he remained in the French occupation zone, dying in a Konstanz hospital on 28 September 1947. He had been a top official at the German Foreign Ministry in 1938, at the time of the German invasion of Czechoslovakia. He had been scheduled to appear at a forthcoming trial as a witness on German-Italian negotiations during the build up to that invasion. His death from lung cancer meant that he was spared the court appearance.