David Passaro, a former Special Forces medic, was convicted on Thursday of assault causing serious injury and three counts of assault.
Passaro, 40, was the first civilian to be charged with abusing a detainee in the US wars in Iraq and Afghanistan.
During a trial that began on August 7, prosecutors said Passaro beat Abdul Wali so badly he pleaded to be shot to end his pain.
Wali died of his injuries two days after the interrogation in June 2003.
Prosecutors had charged Passaro with two counts of assault with a dangerous weapon - a black metal flashlight - with intent of bodily harm, and two counts of assault resulting in serious bodily injury.
Those counts included a potential sentence of up to 40 years in prison, but the jury convicted him of the lesser charges, an option given by US district court judge Terrence Boyle during jury instructions.
The indictment said Passaro worked at a US military base in Afghanistan that was frequently subjected to rocket attacks and Wali was a suspect in the attacks.
Passaro's lawyers portrayed their client as a good soldier who went out of his way to offer care to Wali.
They said Passaro even performed mouth-to-mouth resuscitation in an unsuccessful bid to revive him.
Guidelines given to interrogators have been an issue since a scandal broke at Iraq's Abu Ghraib prison in 2004.
Prisoners released from the US naval base at Guantanamo Bay in Cuba, where the United States is keeping suspected Taliban and al-Qaeda members, also say they were tortured or abused.
Critics say US government guidelines on what constitutes torture issued since the September 11 attacks have created a climate in which abuse has flourished.
Passaro, who was not charged with the detainee's death, faces up to eleven and a half years in prison. No sentencing date has been set.