Born in Schaerbeek, Paul-Henri was the grand-son of the Liberal politician Paul Janson and nephew of another Liberal politician, Paul-Emile Janson, who was briefly Prime Minister of Belgium from 1937 to 1938. His mother, Marie Janson, was the country's first female Senator. During World War I, he lied about his age to be accepted in the Army; he subsequently spent two years as a German prisoner of war.
Spaak studied law in Brussels and became a member of the Socialist Belgian Labour Party in 1920. Elected deputy in 1932, in 1935 he entered the government led by Paul Van Zeeland as Minister of Transports. He was several times Minister of Foreign Affairs and Prime Minister of Belgium:
- February 1936 - May 1938: Mininster of Foreign Affairs in the second cabinet led by Paul Van Zeeland
- May 1938 - February 1939: Prime Minister
- September 1939 - February 1945: Minister of Foreign Affairs in the 6 government coalitions led by Hubert Pierlot
- February 1945 - January 1946: Minister of Foreign Affairs in the 2 government coalitions led by Achille Van Acker
- March 13 1946 - March 31 1949: Prime Minister and Minister of Foreign Affairs in the shortest-lived Belgian government ever
- March 1949 - August 1946: Minister of Foreign Affairs in the third coalition led by Achille Van Acker
- August 1946 - March 1947: Minister of Foreign Affairs in the government coalition led by Camille Huysmans
- March 1947 - August 1949: Prime Minister and Minister of Foreign Affairs
- April 1954 - June 1958: Minister of Foreign Affairs in the fourth coalition led by Achille Van Acker
- April 1961 - July 1965: Minister of Foreign Affairs in the coalition government led by Theo Lefèvre
- July 1965 - March 1966: Minister of Foreign Affairs in the coalition government led by Pierre Harmel
Spaak gained international prominence in 1945, when he was elected chairman of the first session of the General Assembly of the United Nations. During the third session of the UN General Assembly in Paris, Spaak apostrophized the Russian delegation with the famous words: "Messieurs, nous avons peur de vous" ( Sirs, we are afraid of you). In 1956, he was chosen by the Council of the North Atlantic Treaty Organisation to succeed Lord Ismay as Secretary General. He held this office from 1957 until 1961, when he was succeeded by Dirk Stikker. Spaak was also instrumental in the choice of Brussels as the new seat of the Alliance's HQ in 1966.
Paul-Henri Spaak retired from politics in 1966. He was member of the Royal Belgian Academy of French Language and Literature. In 1969, he published his memoirs titled Combats inachevés (Unfinished struggles, 2 volumes). Spaak died aged 73, on July 31, 1972 in his home in Braine-l'Alleud near Brussels, and was buried at the Foriest graveyard in Braine-l'Alleud.