Sgt. Lawrence G. Hutchins III, 23, had been charged with premeditated murder but premeditation was stricken from the verdict that was returned by a military jury.
Hutchins was also convicted of conspiracy to commit murder, making a false official statement and larceny. He was acquitted of kidnapping, assault and housebreaking.
There is no mandatory minimum sentence for unpremeditated murder. The maximum is life in prison without parole.
Hutchins stood rigidly and stared straight ahead in the silent courtroom as the verdict was read.
The victim was pulled from his home in April 2006 and shot in a ditch, with an AK-47 and shovel placed nearby to make him look like an insurgent planting a bomb, according to the prosecution.
In another base courtroom, meanwhile, a sentencing hearing was under way for a member of the squad convicted on Wednesday of conspiracy and lesser crimes but acquitted of premeditated murder and kidnapping.
Cpl. Marshall Magincalda, 24, faced up to life in prison, but no mandatory minimum sentence. He was also found guilty of larceny and housebreaking, and cleared of making a false official statement.
Magincalda was not accused of firing any shots, but was charged with murder for participating in the plot.
All eight members of the squad were initially charged with murder and kidnapping.
Four lower-ranking Marines and a Navy corpsman cut deals with prosecutors in exchange for their testimony and received sentences ranging from one to eight years in prison.
A jury last month acquitted another corporal of murder but convicted him of conspiracy to commit murder and kidnapping. According to testimony, Cpl. Trent Thomas had greater involvement in the killing than Magincalda. Thomas was sentenced to a reduction in rank and a bad-conduct discharge but no prison time.
The squad was pulled from the battlefield after the slaying in the Anbar province town.
Prosecutors said that during a nighttime patrol, the squad hatched a plan to kidnap a suspected insurgent from his house and kill him. When they could not find him, they instead kidnapped a man from a neighboring house, dragged him to a hole and shot him, the prosecution said.
Several witnesses testified the plot was born out of frustration after suspected insurgents kept evading prosecution.
Prosecutors singled out Hutchins as the ringleader, and testimony from the more junior troops seemed to focus on him and, to a lesser extent, his two corporals.
Magincalda was accused of being part of the four-man "snatch team" that seized the victim from his home. His attorneys contended he wanted no part in the conspiracy and told the squad he would not shoot anyone.
Hutchins' attorneys claimed he participated in the plot because his officers had set a poor leadership example and given approval for Marines to use violence in capturing and interrogating suspected insurgents.