From 1931, Jablonski 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 Jablonski 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).
From 1947 to 1972, he was a deputy to the Sejm. In 1948 he was appointed professor at Warsaw University (from 1952 member of Polish Academy of Sciences – PAN). Between 1948 and 1981, Henryk Jablonski was in the Central Committee of the Polish United Workers' Party (KC PZPR).
In 1972, based on the initiative of Edward Gierek, Jablonski was chosen the de jure leader (chairman of the Council of State) of the People's Republic of Poland. From 1976 to 1983 he was a chief of the Front of National Unity (FJN, Front Jednosci Narodu). From 1983 to 1990 he was chief of ZBoWiD, the Society of Fighters for Freedom and Democracy, an organisation of war veterans. In 1985 he was replaced as head of state by General Wojciech Jaruzelski.