As a pre-medical student at Mahidol University, Prommin played a leading role in the student democratic movement after the mass protests against the military regime in 1973. "When I was twenty, I assumed a critical position that resulted in the loss of lives of several of my friends. It is still in my heart. So I don't want to relive that haunting experience again", said Prommin. His role in the burgeoning democratic movement continued until the 6 October 1976 massacre, when the military brutally cracked down on students at Thammasat University.