Born to a Saxon family in Reußmarkt, Transylvania, son of a doctor and pharmacist, Capesius began his academic studies in 1924 at the University of Cluj after graduating from high school.

After the start of World War II in 1939, Capesius joined the Romanian army and rose to the rank of captain while serving at a military hospital's pharmacy.

In January 1942, for reasons not made clear in his military records, he was granted leave to restart his civilian job as a national sale representative for IG Farben and Bayer.

[5] Capesius worked closely with Josef Mengele and together they were heavily involved in the selection of inmates for the gas chamber beginning in the spring of 1944 when the Hungarian and Transylvanian Jews were sent to the camp.

[8] During his time at Auschwitz, Capesius stole valuables from the personal belongings of arriving Jews at the railhead and also secreted away gold pulled from the dental fillings of corpses.

During a visit to Munich in 1946, Capesius was recognized by a former Auschwitz political prisoner, Leon Czekalzki, and was arrested at the main train station, Hauptbahnhof, by American military police and interned in the camps of Dachau and Ludwigsburg.

[11] A camp survivor, Hermann Langbein, and Germany's first postwar Jewish prosecutor, Fritz Bauer, were persistent in trying to compile enough evidence to make a case against Capesius.