The court heard last week that Howes was shot in the head after receiving a final meal of fruit, which he ate on the floor by the light of car headlights. Little is known about the murder of the interpreter who was killed in another village.
"We are pleased that the murderers of Christopher and Hout have been brought to account," Patricia Phillips, 'Howes' sister, said in a statement distributed through Mines Advisory Group representatives.
"At last justice has been done."
However, Chhun Kham, the translator's widow, said: "It is not justice for me that I have been waiting for many years and the verdict today does not compensate enough the death of my husband."
The two men's remains were found in 1998 as the Khmer Rouge collapsed after two decades of bloody civil war against government forces. The movement had ruled Cambodia from 1975 to 1979, forcing millions of people into forced labour and collective farms, before being forced from power by a Vietnamese invasion.
The defendants had pleaded not guilty when they went on trial in Phnom Penh last week on charges of premeditated murder and illegal confinement.
Three of the five defendants testified that two other Khmer Rouge fighters, now believed to be dead, were responsible for the murders.
One of the defendants, Khem Ngun, admitted that he was responsible for transporting Howes to the execution site, but denied he had prior knowledge of a plan to kill him.
But Khem was jailed for 20 years, along with Loch Mao and Put Lim. Sin Dorn was convicted of being involved in the early stages of the abductions and was sentenced to 10 years.
"The verdict is unjust for me. I did not kill them," Put told reporters.
The fifth defendant was acquitted.