Steven Green was said by his lawyer to have gone 'mad'
Private Steven Green was found guilty of 16 counts of murder, rape and obstructing justice at a court in Kentucky.
Three other US soldiers were given life sentences for their part in the March 2006 atrocity.
Geeen confessed to the murders when army investigators were called to the scene the next day.
But the involvement of the others only came to light when stress counsellors talked to their squad several months later.
The chief prosecutor said the pressures of serving in Iraq and the tragic losses suffered by Green's unit there did not excuse his actions.
"The evidence in this case suggests the defendant was acting purposefully and intentionally with full knowledge of what he was doing," she told the jury.
She said Green and other soldiers changed their clothes and disguised their appearance to throw suspicion on insurgents.
They also burned the body of the 14-year-old girl, Abeer al Janabi, and their own clothes to destroy any evidence that might link them to the crime, she said.
Green's defence lawyer told the jury the stresses of war had cost him his sanity. "Madness. That's the only possible word," he said.
He pointed out Green had been diagnosed as having Combat Operational Stress Disorder three months before the attack.
He also contended the other soldiers involved, Privates James Barker and Paul Cortez, took advantage of his mental condition to carry it out.
The pair told the jury they raped Abeer while Green took her six-year-old sister and her mother and father to another room where he shot them dead.
After raping Abeer himself, Green placed a pillow over the girl's face and shot her three times with an AK-47.
Private Jesse Spielman also received a life sentence for raping the teenager and participating in the murders.
A fifth soldier, Private Bryan Howard, was given a 27-month jail sentence for acting as a lookout.
The jury heard the crime plot was hatched as the men drank whisky during a game of cards at the checkpoint south of Baghdad where they were based.