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 Consul 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.