After graduating at the top of his class in 1921, Diệm followed in the footsteps of his eldest brother, Ngô Đình Khôi, joining the civil service in Thừa Thiên as a junior official.
Starting from the lowest rank of mandarin, Diệm steadily rose over the next decade.
He first served at the royal library in Huế, and within one year was the district chief in both Thừa Thiên and nearby Quảng Trị province, presiding over seventy villages.
Diệm was promoted to be a provincial chief (Tuần phủ) in Ninh Thuận at the age of 28, overseeing 300 villages.

During his career as a mandarin, Diệm was known for his workaholism and incorruptibility, and as a Catholic leader and nationalist.
Catholic nationalism in Vietnam during the 1920s and 1930s facilitated Diệm's ascent in his bureaucratic career.
Diệm's rise was also facilitated through Ngô Đình Khôi's marriage to the daughter of Nguyễn Hữu Bài (1863–1935), the Catholic head of the Council of Ministers at the Huế court and also supported the indigenization of the Vietnamese Church and more administrative powers to the monarchy.
Nguyễn Hữu Bài was highly regarded among the French, and Diệm's religious and family ties impressed him and he became Diệm's patron.
The French were impressed by his work ethic but were irritated by his frequent calls to grant more autonomy to Vietnam.
Diệm replied that he contemplated resigning but encouragement from the populace convinced him to persist.
In 1925, he first encountered communists distributing propaganda while riding horseback through the region near Quảng Trị.
Revolted by calls for violent socialist revolution contained in the propaganda leaflets, Diệm involved himself in anti-communist activities for the first time, printing his own pamphlets.

In 1929, he was promoted to the governorship of Bình Thuận Province and was known for his work ethic.
In 1930 and 1931, he helped the French suppress the first peasant revolts organized by the communists.
According to Fall, Diệm put the revolution down because he thought it could not sweep out the French, but might threaten the leadership of the mandarins.
In 1933, with the ascension of Bảo Đại to the throne, Diệm accepted Bảo Đại's invitation to be his interior minister following lobbying by Nguyễn Hữu Bài.
Soon after his appointment, Diệm headed a commission to advise on potential administration reforms.
After calling for the French to introduce a Vietnamese legislature and many other political reforms, he resigned after three months in office when his proposals were rejected.
Diệm denounced Emperor Bảo Đại as "nothing but an instrument in the hands of the French," and renounced his decorations and titles from Bảo Đại.
The French then threatened him with arrest and exile.

For the next decade, Diệm lived as a private citizen with his family in Huế, although he was kept under surveillance.
He spent his time reading, meditating, attending church, gardening, hunting, and in amateur photography.
Diệm also conducted extensive nationalist activities during those 21 years, engaging in meetings and correspondence with various leading Vietnamese revolutionaries, such as his friend, Phan Bội Châu, a Vietnamese anti-colonial activist, whom Diệm respected for his knowledge of Confucianism and argued that Confucianism's teachings could be applied to a modern Vietnam.
With the start of the World War II in the Pacific, seeing an opportunity for Vietnam to challenge French colonization, he attempted to persuade the Japanese forces to declare independence for Vietnam in 1942 but was ignored.
Diệm also tried to establish relationships with Japanese diplomats, army officers, and intelligence operatives who supported Vietnam's independence.
In 1943, Diệm's Japanese friends helped him to contact Prince Cường Để, an anti-colonial activist, who was in exile in Japan.
After contacting Cường Để, Diệm formed a secret political party, the Association for the Restoration of Great Vietnam (Việt Nam Đại Việt Phục Hưng Hội), which was dominated by his Catholic allies in Hue.
When its existence was discovered in the summer of 1944, the
After graduating at the top of his class in 1921, Diệm followed in the footsteps of his eldest brother, Ngô Đình Khôi, joining the civil service in Thừa Thiên as a junior official. Starting from the lowest rank of mandarin, Diệm steadily rose over the next decade. He first served at the royal library in Huế, and within one year was the district chief in both Thừa Thiên and nearby Quảng Trị province, presiding over seventy villages. Diệm was promoted to be a provincial chief (Tuần phủ) in Ninh Thuận at the age of 28, overseeing 300 villages.
During his career as a mandarin, Diệm was known for his workaholism and incorruptibility, and as a Catholic leader and nationalist. Catholic nationalism in Vietnam during the 1920s and 1930s facilitated Diệm's ascent in his bureaucratic career. Diệm's rise was also facilitated through Ngô Đình Khôi's marriage to the daughter of Nguyễn Hữu Bài (1863–1935), the Catholic head of the Council of Ministers at the Huế court and also supported the indigenization of the Vietnamese Church and more administrative powers to the monarchy. Nguyễn Hữu Bài was highly regarded among the French, and Diệm's religious and family ties impressed him and he became Diệm's patron. The French were impressed by his work ethic but were irritated by his frequent calls to grant more autonomy to Vietnam. Diệm replied that he contemplated resigning but encouragement from the populace convinced him to persist. In 1925, he first encountered communists distributing propaganda while riding horseback through the region near Quảng Trị. Revolted by calls for violent socialist revolution contained in the propaganda leaflets, Diệm involved himself in anti-communist activities for the first time, printing his own pamphlets.
In 1929, he was promoted to the governorship of Bình Thuận Province and was known for his work ethic. In 1930 and 1931, he helped the French suppress the first peasant revolts organized by the communists. According to Fall, Diệm put the revolution down because he thought it could not sweep out the French, but might threaten the leadership of the mandarins. In 1933, with the ascension of Bảo Đại to the throne, Diệm accepted Bảo Đại's invitation to be his interior minister following lobbying by Nguyễn Hữu Bài. Soon after his appointment, Diệm headed a commission to advise on potential administration reforms. After calling for the French to introduce a Vietnamese legislature and many other political reforms, he resigned after three months in office when his proposals were rejected. Diệm denounced Emperor Bảo Đại as "nothing but an instrument in the hands of the French," and renounced his decorations and titles from Bảo Đại. The French then threatened him with arrest and exile.
For the next decade, Diệm lived as a private citizen with his family in Huế, although he was kept under surveillance. He spent his time reading, meditating, attending church, gardening, hunting, and in amateur photography. Diệm also conducted extensive nationalist activities during those 21 years, engaging in meetings and correspondence with various leading Vietnamese revolutionaries, such as his friend, Phan Bội Châu, a Vietnamese anti-colonial activist, whom Diệm respected for his knowledge of Confucianism and argued that Confucianism's teachings could be applied to a modern Vietnam. With the start of the World War II in the Pacific, seeing an opportunity for Vietnam to challenge French colonization, he attempted to persuade the Japanese forces to declare independence for Vietnam in 1942 but was ignored. Diệm also tried to establish relationships with Japanese diplomats, army officers, and intelligence operatives who supported Vietnam's independence. In 1943, Diệm's Japanese friends helped him to contact Prince Cường Để, an anti-colonial activist, who was in exile in Japan. After contacting Cường Để, Diệm formed a secret political party, the Association for the Restoration of Great Vietnam (Việt Nam Đại Việt Phục Hưng Hội), which was dominated by his Catholic allies in Hue. When its existence was discovered in the summer of 1944, the 
Ngo Dinh Diem