In 1945 he was imprisoned and exiled to China following clonflicts with anti-French communist forces that were gaining power in Vietnam. After his release, he refused to join in the brief post-war government of Ho Chi Minh and went into exile in the U.S.A. He returned to be appointed Prime Minister of South Vietnam by the Emperor in 1954 following the French withdrawal. He rejected the Geneva Accord calling for unification and elections in 1956, using a 1955 referendum he abolished the monarchy and declared himself President of South Vietnam.
His rule was authoritarian, corrupt and repressive and soon alienated the people, exemplified by the imprisonment and execution of hundreds of Buddhists. His refusal to institute land reforms probably contributed to increasing popular support for Ho Chi Minh, but he retained American support despite his erratic and independent traits. When the regime turned on a protest by Buddhist monks in June 1963, he lost American aid too; some monks had immolated themselves in protest. The U.S. supported a military coup d'etat of ARVN generals that overthrew the government and executed Ngo, his younger brother (Dinh Nhu) and some others in November.