Modzelewski was born in to the family of a railroad worker. He was a member of the
Social Democracy of the Kingdom of Poland and Lithuania and
Communist Party of Poland. From 1923 to 1937, he was a member of the
French Communist Party and even joined its Central Committee. In 1937 he moved to the Soviet Union and was arrested by
NKVD in the same year in the
Great Purge. Despite torture he refused to give false confession and was released in 1939. During the
Second World War he joined the
Union of Polish Patriots and the
Central Bureau of Polish Communists and became the first director of Polpress, a precursor to the
Polish Press Agency. In May 1943 he was involved in the formation of the
Polish 1st Tadeusz Kościuszko Infantry Division in the camp in
Seltsy. At the rank of captain he became a lecturer in the Political Section of the Division. He joined the
Polish Workers' Party in 1944 (and later its successor, the
Polish United Workers' Party) and eventually became the member of its
Central Committee. On January 2, 1945, he was appointed Polish ambassador to the USSR. He held this position until June 28, 1945, when the Moscow Conference on the establishment of the Provisional Government of National Unity ended. After returning to Poland, he became Undersecretary of State in the Ministry of Foreign Affairs, effectively heading the ministry in the TRJN between 1945 and 1947. He was a member of the Polish delegation to the
Potsdam Conference. He was also a Polish delegate to the session of the Preparatory Commission of the United Nations. Involved in the work of the Slavic Committee in Poland. Minister of Foreign Affairs in 1947–1951. From 1951, Rector of the Institute for the Education of Scientific Personnel. From 1948, member of the Polish United Workers' Party and at the same time member of the Central Committee of the PZPR. In 1951, he defended his doctoral thesis in philosophy. From 1951, full professor, and from 1952, full member of the
Polish Academy of Sciences. Between 1947 and 1952 he was a member of
Legislative Sejm and in the years 1952–1954 a member of the
Polish Council of State. He died on June 18, 1954. He was buried at the Powązki Military Cemetery in Warsaw. He was the adoptive father of
Karol Modzelewski. ==Awards and decorations==