Thieu was appointed as the head of the National Military Academy of the Republic of Vietnam in 1956. Thieu held the position for 4 years. During his term in service he was sent to the United States for further military training in 1957 and 1960. 

On 3 September 1967, Thieu ran for, and was successfully elected, as the second president of the Republic of South Vietnam with Nguyen Can Ky as his running mate. Thieu took 34% of the vote and held the position for two terms. 

In the 1971 re-election, with Thieu's reputation for corruption, his political opponents believed the poll would be rigged, so they declined to run. As a result, Thieu was the only candidate who ran for the election. The result was that Thieu was easily re-elected, receiving 94% of the vote from an 87% turn-out.

Vice President Tran Van Huong took over the role of President, but it was too late. On  30 April, the North Communists advanced into the South Vietnamese capital of Saigon marking their victory as well as the end of the war, uniting North and South Vietnam into one country under the Communist Party. 

Thieu's decision to surrender the coastal cities (which included his own hometown) produced immense criticism. It's also understood that if South Vietnam continued to fight, they would not have been able to hold the coastal cities since they had limited equipment to fight which would have resulted in even greater casualties. Thieu's decision was made with the intention to only protect troops as well as keeping the force in one place, protecting the main cities with resources that were deemed to be useful to the war. While it is not entirely true, Thieu came to be seen as a puppet figure of the United States by North Vietnam. Yet Thieu often opposed US proposals that would have put South Vietnam in a disadvantaged position. He always believed - and was proven correct - that Hanoi's goal was total victory, not a negotiated settlement. Under relentless pressure from Washington,  he reluctantly  signed the Paris Peace Agreement in 1973. 

Thieu later tried to seek aid from the US when the North started attacking the South again but was rejected. His legacy as the man who presided over the fall of South Vietnam cannot be separated from decisions of the American government. Thieu was seen as a President who cared for his citizens and the interests of his country. He did not, however, have the ability to run the country even prior to the US withdrawal of troops in 1973. 

In conclusion, Thieu was popular among both South Vietnamese citizens, and many international allies. During his time in office, he was valued for his intellect, and ability to speak fluent French and English. Despite these qualities, he is judged as a failure in the American war against communism.