Born in Gnesen, Jahnke immigrated to the United States in 1899, became a naturalized citizen, and served in the U.S. Marines in the Philippines. From August 1914, under the command of the German Counsel General Franz Bopp, Jahnke performed various intelligence and sabotage operations for the German Admiralty from his San Francisco base. He and his assistant Lothar Witzke were responsible for the March 1917 munitions explosion at the Mare Island Naval Shipyard in San Francisco, "likely" responsible for the Black Tom explosion in Jersey City, New Jersey, and are suspected of other explosions and of fomenting labor strikes. When the United States entered the war in April 1917, Jahnke and Witzke moved their operations to Mexico City.
According to the Senate testimony of intelligence officer and double agent Dr. Paul Altendorf, who was undercover in Mexico City with the U.S. Military Intelligence Corps from 1917 through April 1919, Jahnke had schemed a Mexican attack on the United States. An army of 45,000 men, funded by ambassador von Eckardt and trained by German reservists, would march against the U.S. in 1918 and "arouse the Negroes to civil war."
Back in Germany in the late 1930s, Jahnke established the "Jahnke Büro", essentially a small private intelligence service reporting up to Rudolf Hess. There is speculation that Jahnke was somehow involved in Hess's still-puzzling flight to Scotland in May 1941; true or not, he fell out of favor with Ribbentrop and the Gestapo and his accumulated files were confiscated. Later in the war, Jahnke served as intelligence advisor to Walter Schellenberg. He and his wife were captured by Soviet SMERSH agents in April 1945, interrogated, and executed.
- State Secrets: The Kent-Wolkoff Affair By Bryan Clough, page 224
- World War One By Priscilla Mary Roberts, page 1606
- World War I: encyclopedia. S - Z, Volume 4 edited by Spencer Tucker, page 1033
- Investigation of Mexican affairs: Hearing before a subcommittee of the Committee on Foreign Relations, United States Senate, Sixty-sixth Congress, Volume 1, 1919, pages 459-461, full text via Google Books
- "Insists Carranza Aided German Plot, New York Times, September 3, 1919
- State Secrets: The Kent-Wolkoff Affair By Bryan Clough, page 228
- Hitler's last chief of foreign intelligence: Allied interrogations of Walter Schellenberg, page 21
- Reinhard R. Doerries: Tracing Kurt Jahnke: Aspects of the Study of German Intelligence. In: George O. Kent (Hrsg.): Historians and Archivists. (Fairfax, VA, 1991), 27–44.