Po Hnit had collected a personal library of various American and British books and cultivated a reading habit among his children. As a result, Thant became an avid reader and his school friends nicknamed him "The Philosopher". Apart from reading, he enjoyed various Sports including hiking, swimming and playing chinlone. He went to the National High School in Pantanaw. At the age of eleven, Thant participated in strikes against the University Act of 1920. He dreamed of becoming a Journalist and surprised the family by writing an article for the Union of Burma Boy Scouts magazine. When Thant was fourteen, his father died and a series of inheritance disputes forced Thant's mother, Nan Thaung, and her four children into difficult financial times.

After the death of his father, Thant believed he would not be able to complete a four-year degree and instead worked for a two-year teaching certificate at Rangoon University in 1926. As the oldest son, he had to fulfill his filial duties and responsibilities of the family. At university, Thant, together with Nu, the Future Prime Minister of Burma, studied history under D. G. E. Hall. Nu was told by a distant mutual relative to take care of Thant and the two soon became close friends. Thant was elected joint secretary of the University Philosophical Association and secretary of the Literary and Debating Society. In Rangoon, Thant met J.S. Furnivall, the founder of The Burma Book Club and The World of Books magazine, in which Thant regularly contributed. Promising a good post, Furnivall urged Thant to complete four-year university course and join Civil Service but Thant refused. After earning the certificate, he returned to Pantanaw to teach at the National High School as a senior Teacher in 1928. He contacted Furnivall and Nu regularly, writing articles and participating in The World of Books translation competitions.

In 1931, Thant won first place in All Burma Teachership Examination and became the school's headmaster by the age of twenty-five. Urged by Thant, his friend Nu took the local superintendent of schools position. Thant regularly contributed to several newspapers and magazines under the pen name "Thilawa" and translated a number of books, including one on the League of Nations. His major influences were Sir Stafford Cripps, Sun Yat-sen and Mahatma Gandhi. In the days of tense political climate in Burma, Thant stood moderate grounds between fervent nationalists and British loyalists.

During the World War II, the Japanese occupied Burma from 1942 to 1945. They brought Thant to Rangoon to lead the Educational Reorganizing Committee. However, Thant did not have any real power and returned to Pantanaw. When the Japanese ordered making Japanese compulsory in Pantanaw high schools, Thant defied the orders and cooperated with the growing anti-Japanese resistance.

In 1948, Burma gained independence from the United Kingdom. Nu became the prime minister of the newly independent Burma and appointed Thant as Director of broadcasting in 1948. By then, civil war had broken out. The Karen insurgency began and Thant risked his life to go to Karen camps to negotiate for peace. The negotiations broke down, and in 1949 the advancing insurgents burned his hometown, including his house. The insurgents pushed the front to within four miles of the capital Rangoon before they were beaten back. In the following year, Thant was appointed secretary to the government of Burma in the Ministry of Information. From 1951 to 1957, Thant was secretary to the prime minister, writing speeches for U Nu, arranging his foreign travel, and meeting foreign visitors. During this entire period, he was U Nu's closest confidant and advisor.

He also took part in a number of international conferences and was the secretary of the 1955 Bandung Conference in Indonesia, which gave birth to the Non-Aligned Movement. From 1957 to 1961, he was Burma's permanent representative to the United Nations and became actively involved in negotiations over Algerian independence. In 1961, Thant was named Chairman of the UN Congo Commission. The Burmese government awarded him the title Maha Thray Sithu as a commander in the order of Pyidaungsu Sithu.

In Israel, his abrupt unilateral withdrawal of UNEF without any diplomatic process or wider consultation was regarded as a violation of United Nations assurances and commitments given to Israel in 1957, on the basis of which Israel had withdrawn from Sinai and Gaza at that time, and it "thereafter inspired Israel's refusal to place her vital interests again in United Nations hands".

Thant was generally reluctant to receive prizes and honors due to his own humility as well as publicity associated with them. He declined Burma's second highest honor awarded to him by U Nu's government in 1961. When he was informed that the 1965 Nobel Prize would instead go to UNICEF due to Chairman Gunnar Jahn's veto, Thant, according to Walter Dorn, "recorded his pleasure". However, he did accept the Jawaharlal Nehru Award for International Understanding in 1965, the Gandhi Peace Award in 1972, and more than two dozen honorary degrees.

Although a manifest pacifist and a devout Buddhist, Thant did not hesitate to use force when required. During the Congo Civil War in 1962, Katangan secessionists led by Moise Tshombe repeatedly attacked UN Operation in the Congo forces (ONUC). In December 1962, after ONUC suffered a sustained four-day attack in Katanga, Thant ordered the "Operation Grandslam" to gain "complete freedom of movement for ONUC all over Katanga." The operation proved to be decisive and ended the secessionist insurgency once and for all. By January 1963, the secessionist capital Elizabethville was under full UN control.

In April 1964, Thant accepted the Holy See’s designation of itself as a permanent observer. There appeared to be no involvement of the General Assembly or the UN Security Council in the decision.

For his role in defusing the Cuban crisis and other peacekeeping efforts, the Norwegian Permanent Representative of the United Nations informed Thant that he would be awarded the 1965 Nobel Peace prize. He humbly replied, "Is not the Secretary-General merely doing his job when he works for peace?" On the other hand, Chairman Gunnar Jahn of the Nobel Peace prize committee lobbied heavily against giving Thant the prize which was at the last minute awarded to UNICEF. The rest of the committee all wanted the prize to go to Thant. The disagreement lasted three years and in 1966 and 1967 no prize was given, with Gunnar Jahn effectively vetoing an award to Thant. Outraged, Thant's undersecretary and Nobel Prize laureate Ralph Bunche remarked Gunnar Jahn's decision as "gross injustice to U Thant."

Thant was re-appointed secretary-general of the United Nations by the General Assembly on 2 December 1966, on the unanimous recommendation of the Security Council. His term of office continued until 31 December 1971, when he retired. During his time in office, he oversaw the entry into the UN of dozens of new Asian and African states and was a firm opponent of apartheid in South Africa. He also established many of the UN's development and environmental agencies, funds and programmes, including the UN Development Programme (UNDP), the UN University, the United Nations Conference on Trade and Development, United Nations Institute for Training and Research (UNITAR), and the UN Environmental Programme. The Six Day War between Arab countries and Israel, the Prague Spring and subsequent Soviet invasion of Czechoslovakia, and the Indo–Pakistani War of 1971 leading to the birth of Bangladesh all took place during his tenure as secretary-general.

During his tenure as Secretary-General, Thant followed UFO reports with some interest; in 1967, he arranged for American atmospheric Physicist James E. McDonald to speak before the UN's Outer Space Affairs Group regarding UFOs.

However, Thant's grandson, Thant Myint-U, wrote in the book The River of Lost Footsteps: Histories of Burma that intense animosity between Thant and Ne Win goes back only to 1969, when Ne Win believed Thant was conniving with Nu after Nu denounced Ne Win at a press corp meeting at the UN headquarters. Ne Win told his men to consider Thant as an enemy of the state, despite Thant denouncing Nu's action as inappropriate.

In his farewell address to the United Nations General Assembly, Secretary-General Thant stated that he felt a "great sense of relief bordering on liberation" on relinquishing the "burdens of office". In an editorial published around 27 December 1971, praising Thant, The New York Times stated that "the wise counsel of this dedicated man of peace will still be needed after his retirement". The editorial was titled "The Liberation of U Thant".

During the period of 5–11 December, the student demonstrators also built a temporary mausoleum for Thant on the grounds of the RUSU and gave anti-government speeches. In the early morning hours of 12 December 1974, government troops stormed the campus, killed some of the students guarding the makeshift mausoleum, removed Thant's coffin, and reburied in Kandawmin Garden Mausolea near the Shwedagon Pagoda, where it has continued to lie. Upon hearing of the storming of the Rangoon University campus and the forcible removal of Thant's coffin, many people rioted in the streets of Rangoon. Martial law was declared in Rangoon and the surrounding metropolitan areas. What has come to be known as the U Thant crisis—the student-led protests over the shabby treatment of Thant by the Ne Win government—was crushed by the Burmese government.

After his retirement, Thant was appointed a senior fellow of the Adlai Stevenson Institute of International Affairs. He spent the last years of his life writing and advocating the development of a true global community and other general themes he had tried to promote while he was secretary-general. While serving as secretary-general, Thant lived in Riverdale, Bronx, on a 4.75-acre (1.92 ha) estate near 232nd Street, between Palisade and Douglas Avenues.

In 1978, Thant's memoirs, View from the UN, were posthumously published, initially by Doubleday.

In April 2012, UN Secretary-General Ban Ki-moon paid his respects at U Thant's mausoleum during a visit to Yangon.

In December 2013, in an effort spearheaded by his daughter Aye Aye Thant and his grandson Thant Myint-U, Thant's house in Yangon was being converted into a museum which would feature his photos, works and personal belongings. In October 2013, the building of an U Thant library near his Pantanaw house was underway.