Ngo Dinh Diem is best known to Americans as the man who was the leader of South Vietnam from very soon after the country was created in 1954 until he was assassinated in 1963.
Diem's earlier career was spent mostly in the civil service. This meant that he was a government official working for the French when they held Vietnam as a colony. While working for the French, he rose to high office, becoming the Interior Minister. At that point, he pushed for more Vietnamese autonomy, was denied, and resigned as a consequence.
After resigning in 1933, Diem held no position of power until he became the leader of South Vietnam in 1954. He spent some of that time (1950-4) in exile abroad. After WWII, he tried to gain political support for his idea of anti-communist nationalism. He worked for that goal both at home and while in exile.
Once Diem returned as leader of South Vietnam, he had terrible problems. He was unpopular because he had worked with the French, because he was Catholic, and because he did not try very hard to gain the support of the people. All of this helped to lead to his overthrow and death.