He is professor emeritus of Southeast Asian history at the University of Wisconsin–Madison and a chief senior researcher at the Japanese Institute of Developing Economies.

But following the return of disempowered military dictator Thanom Kittikachorn and the resignation of Prime Minister Seni Pramoj, Thongchai and other student leaders organized a fresh wave of protests centered at Thammasat University.

Thousands of students were arrested, though 19 of the demonstration's "ringleaders" were eventually imprisoned, including Thongchai.

He and his co-defendants were released by a royal amnesty on 16 September 1978, and allowed to return to finish education at Thammasat on the condition that they were not to be involved in further political activities.

Since 1991, he has been a member of the Association for Asian Studies (AAS), chairing its Southeast Asia Council and serving on its executive board in 1996–1997.

[11] From 2017 to 2019, he was a chief senior researcher at the Inter-disciplinary Studies Center of the Institute of Developing Economies (IDE) in Chiba, Japan.

[13] Thongchai's best-known academic work is his book Siam Mapped, published in 1994, which critiqued existing theories of Thai historiography.