Mende was born on 28 October 1916 in Groß Strehlitz, in the Province of Silesia, a province in the Kingdom of Prussia. Today it is Strzelce Opolskie, part of Opole Voivodeship in Poland. He was the third of four children of Max Mende (1885–1943) and his wife Anna (1889–1968), née Krawietz, of Polish descent. He had a seven-year-older brother Walter and a five-year-older sister Amalie. His father was the director of a secondary school ("Volksschule") and, as was usual among Catholics, a supporter of the Centre Party. He grew up in region of Silesia heavily fought over during the Silesian Uprisings of 1919 to 1921 which pinnacled in the Battle of Annaberg (21–26 May 1921). He also experienced the Upper Silesia occupation by British, French and Italian forces, and being governed by an Inter-Allied Committee headed by a French general, Henri Le Rond.