U Thant?s contribution to the world was a legacy of powerful peace-keeping and dynamic leadership based on a philosophy which encompassed the physical, social and spiritual spheres.  A simple teacher from Burma (now Myanmar), U Thant quickly became the most important world official of histime.  As the third Secretary-General of the United Nations, he mediated some of history?s most critical events, including the Cuban Missile Crisis. 
A true man of peace, U Thant strove with every action to impart the idea that world peace could be achieved only when humanity has achieved inner peace.  Drawing from the beliefs of his Buddhist faith, U Thant believed that nurturing the inner aspects of life was equally important as ensuring a high standard of living.  Through his guidance, the UN grew from a fledgling organization to an influential and integral instrument for leadership and change.
U Thant?s thirst for peace transcended any sense of allegience or partiality to a single or group of nations.  This unique universality helped the UN gain the trust of many nations and helped the acceptance of the UN?s innovative social and political programs.  His selfless actions, however, were misunderstood by many.  Undaunted he clung to his ideals, believing that his selfless spirit would ultimately be perceived as the source of his motivation, rather than any ego-based desire for power.  U Thant?s aspiration for the good of the whole possibly cost him the 1965 Nobel Peace Prize as the United States objected to his anti-Vietnam War stance. 
His legacy lives on as the namesake of the U Thant Peace Award as well as the prestigious U Thant Lecture series.
official biography from the United Nations web site.
U Thant, who served as Secretary-General of the United Nations from 1961 to 1971, was chosen to head the world body when Secretary-General Dag Hammarskjold was killed in an air crash in September 1961.
U Thant was born at Pantanaw, Burma, on 22 January 1909, and was educated at the National High School in Pantanaw and at University College, Rangoon.
Prior to his diplomatic career, U Thant’s experience was in education and information work. He served as Senior Master at the National High School, which he had attended in Pantanaw, and in 1931, he became Headmaster after winning first place in the Anglo-Vernacular Secondary Teachership Examination.
He was a member of Burma’s Textbook Committee and of the Council of National Education before World War II, and was an Executive Committee member of the Heads of Schools Association. He was also active as a free-lance journalist.
In 1942, U Thant served for a few months as Secretary of Burma’s Education Reorganization Committee. In the following year, he returned to the National High School as Headmaster for another four years.
U Thant was appointed Press Director of the Government of Burma in 1947. In 1948, he became Director of Broadcasting, and in the following year, he was appointed Secretary to the Government of Burma in the Ministry of Information. In 1953, U Thant became Secretary for projects in the Office of the Prime Minister, and in 1955, he was assigned additional duties as Executive Secretary of Burma’s Economic and Social Board.
At the time of his appointment as Acting Secretary-General of the United Nations, U Thant had been Permanent Representative of Burma to the United Nations, with the rank of Ambassador (1957-1961).
During that period, he headed the Burmese delegations to the sessions of the General Assembly, and in 1959, he served as one of the Vice-Presidents of the Assembly’s fourteenth session. In 1961, U Thant was Chairman of the United Nations Congo Conciliation Commission and Chairman of the Committee on a United Nations Capital Development Fund.
During his diplomatic career, U Thant served on several occasions as Adviser to Prime Ministers of Burma.
U Thant began serving as Acting Secretary-General since 3 November 1961, when he was unanimously appointed by the General Assembly, on the recommendation of the Security Council, to fill the unexpired term of the late Secretary-General, Dag Hammarskjold. He was then unanimously appointed Secretary-General by the General Assembly on 30 November 1962 for a term of office ending on 3 November 1966.
U Thant was re-appointed for a second term as Secretary-General of the United Nations by the General Assembly on 2 December 1966 on the unanimous recommendation of the Security Council (resolution 229, 1966). His term of office continued until 31 December 1971.
U Thant received honorary degrees (LL.D) from the following universities: Carleton University, Ottawa, Canada (25 May 1962); Williams College, Williamstown, Massachusetts (10 June 1962); Princeton University, Princeton, New Jersey (12 June 1962); Mount Holyoke College, South Hadley, Massachusetts (2 June 1963); Harvard University, Cambridge, Massachusetts (13 June 1963); Dartmouth College, Hanover, New Hampshire (16 June 1963); University of California at Berkeley, California (2 April 1964); University of Denver, Denver, Colorado (3 April 1964); Swarthmore College, Swarthmore, Pennsylvania (8 June 1964); New York University, New York (10 June 1964); Moscow University, Moscow, Soviet Union (30 July 1964); Queen’s University, Kingston, Ontario (22 May 1965); Colby College, Waterville, Maine (6 June 1965); Yale University, New Haven, Connecticut (14 June 1965); University of Windsor, Windsor, Ontario, Canada (28 May 1966); Hamilton College, Clinton, New York (5 June 1966); Fordham University, Bronx, New York (8 June 1966); Manhattan College, New York (14 June 1966); University of Michigan, Ann Arbor, Michigan (30 March 1967); Delhi University, New Delhi, India (13 April 1967); University of Leeds, England (26 May 1967); Louvain University, Brussels, Belgium (10 April 1968); University of Alberta, Edmonton, Canada (13 May 1968); Boston Unversity, Boston, Massachusetts (19 May 1968); Rutgers University, New Brunswick, New Jersey (29 May 1968); University of Dublin (Trinity College), Dublin, Ireland (12 July 1968); Laval University, Quebec, Canada (31 May 1969); Columbia University, New York City (3 June 1969); the University of the Philippines (11 April 1970); and Syracuse University (6 June 1970). He also received the following honorary degrees: Doctor of Divinity, The First Universal Church (11 May 1970); Doctor of International Law, Florida International University, Miami, Florida (25 January 1971); Doctor of Laws, University of Hartford, Hartford, Connecticut (23 March 1971); Doctor of Civil Laws degree, honoris causa, Colgate University, Hamilton, New York, (30 May 1971); Doctor of Humane Letters, Duke University, Durham, North Carolina (7 June 1971).
U Thant retired at the end of his second term in 1971 and he died on 25 November 1974 after a long illness. He was 65 years old.
** Formerly known as Burma.