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).

On 1 September 1939, Brandt was appointed by Hitler co-head of the Aktion T4 euthanasia program, with Philipp Bouhler. Additional power was afforded Brandt when on 28 July 1942, he was appointed Commissioner of Sanitation and Health (Bevollmächtigter für das Sanitäts- und Gesundheitswesen) by Hitler and was thereafter only bound by the Führer’s instructions. He received regular promotions in the SS; by April 1944, Brandt was a SS-Gruppenführer in the Allgemeine SS and a SS-Brigadeführer in the Waffen-SS. On 16 April 1945, he was arrested by the Gestapo for moving his family out of Berlin so they could surrender to American forces. Brandt was condemned to death by a military court and then sent to Kiel. He was released from arrest by order of Karl Dönitz on 2 May. He was placed under arrest by the British on 23 May.

Brandt was tried along with twenty-two others at the Palace of Justice in Nuremberg, Germany. The trial was officially titled United States of America v. Karl Brandt et al., but is more commonly referred to as the “Doctors’ Trial”; it began on 9 December 1946. He was charged with four counts:

After a defence led by Robert Servatius, on 19 August 1947, Brandt was found guilty on counts 2-4 of the indictment. With six others, he was sentenced to death by hanging, and all were executed at Landsberg Prison on 2 June 1948. Nine other defendants received prison terms of between fifteen years and life, while a further seven were found not guilty.