Brandt was born in Mulhouse in the then German Alsace-Lorraine territory (now in Haut-Rhin, France) into the family of a Prussian Army officer. He became a medical doctor and surgeon in 1928, specializing in head and spinal injuries. He joined the Nazi Party in January 1932, and first met Hitler in the summer of 1932. He became a member of the SA in 1933 and a member of the SS on 29 July 1934; appointed the officer rank of Untersturmführer. From the summer of 1934 forward, he was Hitler's "escort physician". Karl Brandt married Anni Rehborn (born 1907), a champion swimmer, on 17 March 1934. They had one son, Karl Adolf Brandt (born 4 October 1935).