Piasecki was born on 15 December 1900 in Lwów, Austrian Galicia. He was the son of scouting activist Eugeniusz Piasecki and Gizela Siberfeld, a daughter from a wealthy Jewish family, who converted to Catholic faith during the wedding and took the name Maria Piasecka He was related to the athlete Jadwiga Wajs. In his late teens, Piasecki fought in the Battle of Lemberg (1918), and the Polish-Ukrainian War. He also fought in the Polish-Soviet War of 1920–21 as a volunteer.