The son of Hjalmar Hammarskjöld, prime minister of Sweden (1914–17) and chairman of the Nobel Prize Foundation (1929–47), Dag Hammarskjöld studied law and economics at the universities of Uppsala and Stockholm and taught political economy at Stockholm (1933–36). He then joined the Swedish civil service as permanent undersecretary in the Ministry of Finance and subsequently became president of the board of the Bank of Sweden. From 1947 he served in the Ministry of Foreign Affairs. In 1951 Hammarskjöld was chosen vice chairman of Sweden's delegation to the UN General Assembly, of which he became chairman in 1952. On April 10, 1953, five months after the resignation of Trygve Lie of Norway as secretary-general, Hammarskjöld was elected to the office for a term of five years. In September 1957 he was reelected to another five-year term.

For several years he was most concerned with fighting and threats of fighting in the Middle East between Israel and the Arab states; he and the Canadian statesman Lester Pearson (Pearson, Lester B.) participated in the resolution of the Suez Canal crisis that arose in 1956. Hammarskjöld also played a prominent role in the 1958 crisis in Lebanon and Jordan.

The Belgian Congo became the independent Republic of the Congo (now Democratic Republic of the Congo) on June 30, 1960, and Hammarskjöld sent a UN force to suppress the civil strife that began there soon afterward. In September 1960 his action was denounced by the Soviet Union, which demanded that he resign and that the office of secretary-general be replaced by a three-man board (troika) comprising representatives of the Western, communist, and neutral nations. Soon after, while on a peace mission to President Moise Tshombe of the Congolese province of Katanga, Hammarskjöld was killed in an airplane crash.

As secretary-general, Hammarskjöld is generally thought to have combined great moral force with subtlety in meeting international challenges. He insisted on the freedom of the secretary-general to take emergency action without prior approval by the Security Council or the General Assembly. He also allayed widespread fears that the UN would be completely dominated by its chief source of financial sustenance, the United States. The absence of a major international crisis during the first three years of his secretaryship enabled him to concentrate on quietly building public confidence in himself and his office.