He graduated from the Lycée Albert Sarraut at age 16 and studied overseas at the University of Montpellier in southern France in 1927.
He was known to be one of the active participators in the Nhân Văn affair in the mid-1950s which saw many middle class intellectuals demanding freedom and democracy in communist-led North Vietnam. After he criticised the disastrous land reform campaign in 1956, he was stripped of all positions he held in the government and was forced to retire from practicing law. At age 22, he became the first Vietnamese person to receive two doctorates in France: Juris Doctor (with the dissertation L"individu dans la vieille cité annamite, Code des Lê, Delaware, Droit, Montpellier, Imp de la Presse Montpellier 1932) and a Doctor d"État in Literature (L"Annam dans la littérature française, Delaware, Lettres, Montpellier 1932).
According to the press of the time, a young Vietnamese person with two doctorates was unheard of in the French educational system.
Returning to Vietnam in 1936, Nguyễn Mạnh Tường taught French literature in Hanoi at the Lycée du Protectorat (trường Trung học Bảo hộ or trường Bưởi, since 1945 named Chu Văn An High School). Dissatisfied with prejudiced French policies, he left the school and opened a law firm.
While participating in the Vietnamese resistance against France (see First Indochina War), he worked as a lawyer and taught in Thanh Hóa until the Partition of Vietnam in 1954, when he moved back to Hanoi and became a professor at University of Literature (Đại học Văn khoa, now Vietnam National University, Hanoi). He joined the Vietnamese government delegation at diplomatic talks in Đà Lạt and other peace talks in Beijing and Vienna.