Jonathan "Jack" Idema, a former US Green Beret, was arrested in July along with another ex-serviceman, Brent Bennett, and documentary film-maker Edward Carabello.
Idema and Bennett received 10-year terms while co-defendant Carabello was handed an eight-year term by the special tribunal in Kabul.
They were accused of kidnapping, jailing and torturing at least eight Afghans as part of a "private war on terror".
Idema, who has been described as a bounty hunter by US media, says he was carrying out a genuine anti-terrorist operation in coordination with the US Defence Department and Afghan authorities - a claim denied by both governments.
The two defence lawyers sent to Kabul by the Americans' families said during a procedural hearing on Monday they would prove their clients had questioned genuine terrorist suspects.
Lack of evidence
But the attorneys failed to show evidence to back up claims by Idema that the group were working with the approval of the Defence Department.
The lawyers earlier said they had a videotape in which President Hamid Karzai's brother was seen welcoming Idema at an airport, but later retracted the claim.
International troops deployed in Afghanistan have confirmed they assisted Idema on three separate raids, presuming him to be a member of the US special forces.
The US-led coalition also said it had taken into custody a man presented by Idema as a terrorist suspect, but later released the man.
The case has highlighted the shadowy world of private security contractors in Afghanistan and strengthened calls by rights groups for the US-led military to open its detention centres to independent inspection.