Their remains were found in 1998, the same year Cambodia's civil war ended when the Khmer Rouge movement disintegrated.
All five of the men sentence on Tuesday - Khem Ngun, Puth Lim, Loch Mao, Sin Dorn and Cheap Chet - were arrested over the past year, nearly a decade after a joint investigation into the incident by British and Cambodian police.
Khem Ngun, Puth Lim and Loch Mao, were each sentenced to 20 years in prison, while Sin Dorn was jailed for 10 years.
Iv Kim Sri, the presiding judge, also ordered the men jointly to pay $10,000 to the families of the victims.
The British government and the Mines Advisory Group charity, for which the two dead men worked, welcomed the verdict, saying their families were "extremely satisfied with today's outcome".
"Today, we feel that justice has been done for our two colleagues who were brutally murdered whilst carrying out life-saving work," Lou McGrath, a Mines Advisory Group executive, said in a statement issued at the court.
Bill Rammell, a UK junior foreign minister, said: "I welcome the guilty verdicts and sentences handed down today by a Cambodian court in the trial of those responsible. The verdict brings to an end 12 years of uncertainty for their families."