After the 1962 Burmese coup d'état, Mahn Nyein Maung became involved in underground political activism. He was arrested in July 1967 and imprisoned in the Coco Islands. He escaped in 1970. He was recaptured and imprisoned until April 1973, under a general amnesty. During his time in jail, he became acquainted with Mahn Aung Kyi, the secretary of the All Burma Karen Organization, who would become his political mentor.