Student and assistant lecturer in architecture at the Warsaw University of Technology, Jankowski was mobilized as an officer during German invasion of Poland in 1939. He escaped after having been captured by the Soviet Union forces and joined the Polish Armed Forces in the West. He became a commando (Cichociemny) and in 1942 was parachuted into occupied Poland, where he became an expert document forger (codename "Agaton") for the Polish resistance. In 1944 he took part in the Warsaw Uprising (member of the Batalion Pięść in the Radosław Group), taken prisoner by the Germans after its end, he became an aide-de-camp of the commander of the also interned Armia Krajowa leader, general Tadeusz Bór-Komorowski.