Politically a conservative supporter of Paul von Hindenburg, Gisevius joined the Gestapo shortly after the Nazis came to power, and was later transferred to the police force of the Prussiann Ministry of the Interior. He soon became disillusioned with Hitler and participated in several plots against him. As Vice Consul in Switzerland, he met with Allen Dulles, and agreed to serve as a liaison between him and General Ludwig Beck and Mayor Carl Goerdeler of Leipzig. Upon returning to Germany, he was investigated briefly by the Gestapo, but released. In 1944, after the failed assassination attempt against Hitler, Gisevius fled back to Switzerland.
After the war, Gisevius returned to Germany and 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. His autobiography, Bis zum Bitteren Ende, ("To the Bitter End"), published in 1946, offered a sharp indictment of the Nazi regime, many of whose leading members Gisevius knew personally, as well as of the German people, who, Gisevius claimed, pretended not to know about the atrocities being committed in its name. At the same time, it also offers an exciting insider's account of the German resistance movement.
Hans Gisevius died in Germany in 1974.