The judge also boosted the international community's crusade to bring the crimes of the Khmer Rouge to account during the bloody revolution of the 1970s, in which up to 1 million died, by declaring that two other senior commanders implicated in the killings would now stand trial.
Families of the victims of the three foreign backpackers abducted from a train in a Khmer ambush cheered on hearing the verdict.
"I am very happy. It was worth the trip. I can sleep well tonight," said Dorothy Slater, Mark Slater's mother.
Mr Slater, who was 29, and two other tourists - David Wilson, an Australian, and Jean-Michel Braquet from France - were snatched from a train bound for the coast in a Khmer Rouge ambush in 1994.
Nuon Paet, who denied charges of complicity in murder, kidnapping and terrorism, seldom looked at the row of seats in the crowded court reserved for the families of the victims.
The court was told that the victims were tied up, led down a jungle track and battered to death. Mark Slater was "finished off" with a bullet.
The verdict capped a day of high drama and farce in the first trial of a Khmer Rouge commander since the organisation gained control of Cambodia in 1975.
Diplomats and United Nations observers, in Phnom Penh for the trial, consider the charging of Sam Bith and Chhouk Rin, two Khmer defectors who are now senior government officials, as more significant than a life sentence for Nuon Paet, considered by the international community as a scapegoat.
Mrs Slater cried when the court was unexpectedly shown a videotape of an autopsy of her son's travelling companion, David Wilson,
Even by Cambodian standards, where court proceedings are often chaotic, the trial proved eventful, with the families of the victims often interceding.
Mrs Slater, from Corby, had originally intended to boycott the trial because of accusations by diplomats and UN observers that Nuon Paet is a scapegoat in a showcase trial.
Throughout the trial, Nuon Paet tried to portray himself as the friend of the victims, not their killer. "I gave $500 (pounds 300) to buy food for them. I tried to get them freed. I did not want my leaders to kill them. When they were killed, I asked: `Why did you kill them?'"
He claimed he had only seen the three backpackers for three hours during their six weeks' captivity. The men's bodies were found in Nuon Paet's Phnom Vour "Vine mountain" stronghold when the government launched a rescue mission.
Nuon Paet, the political officer at Vine mountain, claimed he was acting under the orders of two superior officers, Sam Bith, now a general attached to the Defence Ministry, and Chhouk Rin.Reuse content