Jankowski was a student and later, assistant lecturer in architecture at the Warsaw University of Technology in the Second Polish Republic before World War II. He was mobilized as an officer during the German invasion of Poland of 1939. He was captured by the Soviets in eastern Poland but escaped and joined the Polish Armed Forces in the West. Jankowski became an SOE agent (Cichociemny) and in 1942 was parachuted back into occupied Poland, where he became an expert document forger (codename "Agaton", helped set up the Home Army's document forgery department, codenamed "Agaton section") for the Polish resistance. In 1944, he took part in the Warsaw Uprising (commander of 'Agaton Platoon', member of the "Batalion Pięść" in the Radosław Group). He was taken prisoner by the Germans. After the end of the Uprising, he became an aide-de-camp to General Tadeusz Bór-Komorowski, the leader of the Polish Home Army. After the war, he resumed his career as an architect. He took part in the reconstruction of Warsaw; many of his projects took him abroad (to Iraq, Peru, Yugoslavia and Vietnam).