Mende was born in Groß Strehlitz, near Oppolne. 1936 he made his abitur. Afterwards he had to do his working service for the state and then his military service. Up from 1938 he was an officer. At the end of World War II he was major. After he was set free as prisoner of war, his hometown was made Polish. He found a new home in the Ruhr area. Then he studied laws in Cologne and Bonn. In 1949 he finished his doctorade thesis.
In 1946 he joined the FDP. He was Secretary for Intra-German Relations (Bundesminister für gesamtdeutsche Fragen) from 1963 to 1966. In the same time he was Vice Chancellor of West Germany.