After studying law he was employed by the Prussian Ministry of the Interior and was assigned to the newly-formed Gestapo until 1936. Gisevius maintained links with Hans Oster, Hjalmar Schacht and Arthur Nebe, and joined the secret opposition to Hitler, gathering evidence of Nazi crimes. When World War II started, Gisevius joined the Abwehr, and was involved in secret talks with the Vatican. Gisevius met with Allen Dulles in 1943, and agreed to serve as a liaison with the German opposition to Hitler. In 1944, after the failed July 20th assassination attempt against Hitler, Gisevius first hid then fled to Switzerland in 1945.
Gisevius served as a key witness for the prosecution at the Nuremberg Trials in the case against Hermann Göring, his former boss in the Prussian Ministry of the Interior. He also testified against Keitel and Kaltenbrunner. In the cases against Hjalmar Schacht and Wilhelm Frick, he served for the defence.