Michael Pepe was convicted in 2008 but was only sentenced by a federal court in Los Angeles on Friday after a delay following allegations of an affair between a US investigator and a local interpreter.
US Federal Judge Dale Fischer handed Pepe a 210-year sentence after stating the former soldier had shown "absolutely no remorse."
"He went to Cambodia because it was easy to molest little girls there," Fischer said.
"He raped and tortured them ... while maintaining the facade of being interested in the education of Cambodian children."
Fischer said he was imposing a maximum 30 years in jail for each of Pepe's seven convictions, to run consecutively.
"Despite the sincere efforts of defense counsel, the court finds no justification that would allow Mr. Pepe ever to be released from prison," the judge said.
Fischer awarded restitution of $247,000 to organizations that house and cared for the victims. The judge said money from Pepe's military pension would be used to pay for the award.
"As far as I'm concerned, every last dime of it should go to the children," Fischer said.
Pepe, from Oxnard, California, was working part-time as a professor at a Cambodian university in Phnom Penh when he was arrested by the Cambodian National Police in June 2006.