He was part of the anti-French and then anti-Japanese resistance in the 1940s. But he was training for the priesthood at Pali when he joined the anti-French Khmer Issarak in 1964. He soon left Phnom Penh and fell in with the Khmer Rouge. By the late 1960s he was a general and the group's chief-of-staff. He lost the lower part of one leg in fighting around 1970. It is believed that he directed the massive purges that characterised the short-lived Democratic Kampuchea (1975-1979), earning him the nickname 'Butcher'.
After the regime was overthrown in 1979, Ta Mok remained a powerful figure, controlling the northern area of the Khmer Rouge's remaining territory from his base at Anlong Veng. In 1997, following a split in the party, Ta Mok seized control of one faction, naming himself supreme commander. He held the ailing Pol Pot, who died in his custody.
In 1998, following several key defections, Ta Mok was forced to flee Anlong Veng. On March 6, 1999, the general was captured by the Cambodian army near the Thai border and brought to Phnom Penh, where he joined former comrade Kang Kek Ieu ("Duech") at the Military Prosecution Department Detention Facility. Ta Mok was the last leading member of the Khmer Rouge to remain at large in Cambodia; other senior figures had died or already made immunity deals with the government of Hun Sen, including Nuon Chea, Khieu Samphan and Ieng Sary.
In prison his detention period was repeatedly extended without his being brought to trial. Under Cambodian law his trial should have begun within six months of his arrest. Initially charged with membership of an outlawed group and tax evasion, in February 2002 he was charged with crimes against humanity. He is believed to be in relatively poor health, and his only releases from solitary confinment have been hospital visits.