In 1966 Trương Tấn Sang joined the revolution. From 1966 to 1969 he was leader of the Youth-student movement PK 2. From 1969 to 1971, Trương Tấn Sang was Party Committee member, secretary of Youth Union, in charge of the secret guerrilla group in Đức Hòa District in Long An Province. Trương Tấn Sang joined the Communist Party of Vietnam on 20 December 1969. He was jailed by the South Vietnamese government in 1971 and held in prison at Phú Quốc. He was released under the Paris Peace Treaty in 1973. He received his bachelor of law degree in 1990 from the National Academy of Public Administration.

From 1983–86, Trương Tấn Sang headed Ho Chi Minh City (HCMC)’s Forestry Department, as well as the city’s New Economic Zone Development Department. In 1986, he was promoted to the Standing Board of the city’s Party Committee. He became a member of the national party’s Central Committee in 1991. In 1992, he became chairman for HCMC, the number two position in the city government. He joined the Politburo in 1996 as its 14th ranking member. He was party secretary for HCMC, the top position in the city government, from 1996 to 2000. He was promoted to 10th position in the national party at a congress in April 2001. He was also appointed head of the party’s economic commission at this time.

In 2003, he was reprimanded for failing to act in the Năm Cam corruption scandal when he headed the city government. Sang was promoted to fifth position in the party at a congress in April 2006. At this congress, he was also appointed executive secretary of the party’s secretariat, a position which supervises the membership and the internal structure of the party.

Trương Tấn Sang was promoted to the party’s number two slot between congresses in October 2009. His authority soon eclipsed that of General Secretary Nông Đức Mạnh, the only person nominally above Sang in the party hierarchy, according to a leaked diplomatic cable by U.S. Ambassador Michael Michalak.

Sang “assumed many of Manh’s normal responsibilities,” Michalak wrote. At diplomatic meetings, Sang could “comment authoritatively, in detail and without notes,” whereas Mạnh “appeared disengaged” while he read a 30-minute prepared statement “verbatim and in a monotone.” A BBC story described rivalry between Sang and Prime Minister Dũng and described their relationship as “stormy.” Michalak described both Sang and Dũng as “pragmatic” and “market-oriented.” Both are southerners, but traditionally the party’s top slot has gone to a northerner. Nguyễn Phú Trọng, a northerner, was appointed General Secretary of the Communist Party of Vietnam at 11th National Congress held in Hanoi in January 2011. The congress selected a list of Politburo members, and Sang is ranked first on this list. Following the congress, Trong was named the top ranking member of the party’s Secretariat, Vietnam’s most powerful decision-making body, while Sang is ranked second.

The National Assembly of Vietnam elected Trương Tấn Sang as state president on 25 July 2011 with 97.4 percent of the vote. The term of office is five years. Sang told the Assembly that he would defend Vietnam’s independence and territorial integrity, and would resolve the Spratly Islands dispute with China peacefully. As the new president, he will work to set a foundation that will allow Vietnam to be become an industrialized and modernized country by 2020, Sang told the Assembly.

On 25 July 2013, Trương Tấn Sang met with US President Barack Obama to discuss bilateral trade between the U.S. and Vietnam.