[6][7] From 1946 he attended a teacher training college in Phnom Penh, and in the 1950s received a scholarship to study in Paris, where he became a member of a Marxist group of Cambodian students centred on Saloth Sar (alias Pol Pot), Ieng Sary, and Hou Yuon.

In 1962 he was removed from his post at the Institute after being accused of spreading anti-Sangkum ideas amongst the students, though he was permitted to continue teaching and was appointed principal of a high school in Takéo Province.

[10] By 1963, when Saloth Sar took over the Communist Party leadership, a number of leftists were beginning to escape Phnom Penh for the countryside; Sen went underground and was to follow them in 1964, hidden in the car boot of a Chinese diplomatic vehicle.

[10] Sen was initially to join Saloth Sar and other former colleagues at a Vietnamese Communist military base called Office 100, located in the border areas of Cambodia.

[12] After the Cambodian coup of 1970, and the subsequent establishment of the Khmer Republic under Lon Nol, Sihanouk was to join with his former Communist enemies in forming the GRUNK, a Beijing-based government-in-exile.

Son Sen's role meant that he was particularly closely implicated in the many thousands of deaths that occurred in this period due to the arrest and execution of perceived enemies of the regime.

His memoranda to his subordinate Kang Kek Iew, and notes taken during study sessions overseen by Sen for S-21 cadres, reveal his continued interest in history, deeply anti-Vietnamese views, and revolutionary zeal, along with a "schoolmasterish attention to detail".

[15] The Party's paranoia with regard to security, and its obsessive secrecy, led many senior Khmer Rouge cadres to be identified in internal documents by pseudonyms or numbers: Son Sen was often referred to as "Khieu", and his wife as "At".

By August 1985, when the 'retirement' of Pol Pot was officially announced, Son Sen assumed supreme command of the National Army of Democratic Kampuchea, as it was now called.

He was murdered on 15 June 1997, alongside 13 members of his family, including children, on orders of Pol Pot, who at the time was fighting his last battle to regain control of the Khmer Rouge from Ta Mok.