A panel of six Army officers also convicted Chief Warrant Officer Lewis Welshofer Jr., 43, of negligent dereliction of duty. He was acquitted of assault after six hours of deliberations.
Welshofer was accused of putting a sleeping bag over the head of Iraqi Maj. Gen. Abed Hamed Mowhoush, sitting on his chest and using his hand to cover the general's mouth while asking him questions in 2003.
Welshofer, who stood silently and showed no reaction when the verdict was announced, faces a dishonorable discharge and up to three years in prison for negligent homicide and three months for negligent dereliction of duty. Sentencing was scheduled for Monday.
If convicted of the original murder charge, he could have been sentenced to life in prison.
In other developments:
The defense had argued a heart condition caused Mowhoush's death, and that Welshofer's commanders had approved the interrogation technique.
"What he was doing he was doing in the open, and he was doing it because he believed the information in fact would save lives," attorney Frank Spinner said.
Spinner said he was disappointed with the verdict and would decide after sentencing whether to appeal.
"The verdict recognizes the context in which these events took place," he said. "It was a very difficult time in Iraq. There was confusion, and they were not getting clear guidance from headquarters."
Welshofer and prosecutors left without commenting.
During the trial, prosecutor Maj. Tiernan Dolan described a rogue interrogator who became frustrated with Mowhoush's refusal to answer questions and escalated his techniques from simple interviews to beatings to simulating drowning, and finally, to death.
"He treated that general worse than you would treat a dog and he did so knowing he was required to treat the general humanely," Dolan said.
Welshofer used his sleeping bag technique in the presence of lower ranking soldiers, but never in the presence of officers with the authority to stop him, Dolan said.
The treatment of the Iraqi general "could fairly be described as torture," Dolan said.
In an e-mail to a commander, Dolan said, Welshofer wrote that restrictions on interrogation techniques were impeding the Army's ability to gather intelligence. Welshofer wrote that authorized techniques came from Cold War-era doctrine that did not apply in Iraq, Dolan said.
"Our enemy understands force, not psychological mind games," Dolan quoted from Welshofer's message. Dolan said an officer responded by telling Welshofer to "take a deep breath and remember who we are."
The defense urged jurors to consider conditions in Iraq at the time of the interrogation: Soldiers were being killed in an increasingly lethal and increasingly bold insurgency. Welshofer had to make some decisions on his own because guidance was lacking and other techniques weren't working, Spinner said.
Officials believed Mowhoush had information that would "break the back of the whole insurgency," said defense attorney Capt. Ryan Rosauer. They also thought Mowhoush helping to bring foreign fighters into Iraq from across the Syrian border, he said.
Several prosecution witnesses, including one whose identity is classified and who testified in a closed session, had been granted immunity in exchange for their cooperation, Spinner noted. Two soldiers who were initially charged with murder in the case also were given immunity.