Jabłoński was born in Stary Waliszew in the Warsaw Governorate of Congress Poland. From 1931, he was a member of the Polish Socialist Party (PPS). During World War II he fought in the battle of Narvik (Norway) and then joined the French Resistance. In 1945 Jabłoński become a member of the State National Council (KRN), and during the years 1946 and 1948 he had high functions in the PPS (chairman of Central State Committee and its vice-leader). He approved joining the PPS with the Polish Workers' Party (PPR, "Polska Partia Robotnicza").