Charged with spying for the US Central Intelligence Agency, Chhum was incarcerated for 17 months in what he would later describe as a "chicken box" then in a room where he took turns sleeping with 40 other prisoners.

In 1991, following the signature of the Paris Peace Agreements, King Norodom Sihanouk nominated Chau Sen Cocsal Chhum as President of Cambodia's Supreme National Council (SNC).

Le Monde correspondent, Jean-Claude Pomonti, in a personal interview, quoted Chau Sen Cocsal as having "little affinity with the current Cambodian regime", assessing that its leadership "had almost sold Cambodia to the Vietnamese".

On 22 January 2009, at the age of 103, Cambodia's last remaining civil servant from the French Administration, and the Sangkum Reastr Niyum, died peacefully, surrounded by his family.

In an obituary published in the Phnom Penh Post, his family wrote that Chhum should be remembered as "a man considerate of the rights of others, small or tall", who "always had kind words for the anonymous faces tirelessly serving the high and mighty.

"[9] A few years before his death, Chhum had already asked his grandson to convey his apologies to the population of Phnom Penh for the traffic congestion that his long funeral cortege would cause.

"[citation needed] From 10 June 2004, when former Greek Prime Minister Xenophon Zolotas died, until his own death, Chhum was the world's oldest living former head of government.