A trained gynecologist, he turned his medical expertise toward mass sterilization research on concentration camp prisoners, conducting experiments on hundreds of Jewish and Romani women without consent or anesthetic. His work at Auschwitz was part of a broader Nazi program aimed at developing efficient methods of large-scale sterilization. After the war, Soviet imprisonment and a prisoner exchange failed to end his career — he returned to West Germany and resumed medical practice before public pressure from survivors forced his arrest, though he died before facing trial.

A judicial career built on nationalist prosecution provided Hiranuma with both the credentials and the network to ascend to Japan's highest political office during one of its most dangerous periods. His tenure as Prime Minister came as Japan deepened its alignment with fascist powers and tightened authoritarian controls domestically, and his earlier role shaping the justice apparatus gave ideological weight to those structures. He was convicted of war crimes at the International Military Tribunal for the Far East and sentenced to life imprisonment.