U Thant was first appointed as the Acting Secretary-General in 1961 to fill the unexpired term of the late Secretary-General, Dag Hammarskjold.
He was then unanimously appointed Secretary-General by the General Assembly on November 30, 1962 for a term of office ending on November 3, 1966.
During this first term he was widely credited for his role in defusing the Cuban Missile Crisis and for ending the civil war in the Congo.
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, the UN University, UNCTAD, UNITAR and the UN Environmental Programme.
He also led many successful though now largely forgotten mediation efforts, for example in Yemen in 1962 and Baharain in 1968.
Unlike his two predecessors, Thant retired after ten years on speaking terms with all the big powers.
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.
He was widely criticised in the US and Israel for agreeing to pull out UN troops in the Sinai in 1967 in response to a request from Egyptian President Nasser.
In fact, countries such as India and Yugoslavia, which had contributed the troops had already agreed to pull them out, and U Thant was the only world statesman who tried to persuade Nasser not to go to war with Israel, by flying to Cairo in a last minute peace effort.
His once good relationship with the US government deteriorated rapidly when he publicly criticised American conduct of the Vietnam War.
On 23 January 1971, U Thant categorically announced that he would 'under no circumstances' be available for a third term as Secretary-General.