Sgt. Paul E. Cortez, 24, also was given a dishonorable discharge Thursday. He will be eligible for parole in 10 years under the terms of his plea agreement.
Cortez, of Barstow, California, pleaded guilty this week to four counts of felony murder, rape and conspiracy to rape in a case considered among the worst atrocities by U.S. military personnel in Iraq.
In his plea agreement, he said he conspired with three other soldiers from the Fort Campbell-based 101st Airborne Division to rape 14-year-old Abeer Qassim al-Janabi. The girl, her parents and a younger sister were all killed.
Earlier Thursday, tears rolled down Cortez's face as he apologized for the rape and murders. He said he could not explain why he took part, the AP said.
"I still don't have an answer," Cortez told the judge. "I don't know why. I wish I hadn't. The lives of four innocent people were taken. I want to apologize for all of the pain and suffering I have caused the al-Janabi family."
The military judge hearing the case, Col. Stephen R. Henley, issued a sentence of life in prison without parole, the maximum for the charges. Under military law, the defendant is given the lesser sentence unless he violates terms of the plea agreement, which requires Cortez to testify against others charged in the case.
Psychologist Charles Figley testified that Cortez and the other soldiers likely suffered stress brought on by fatigue and trauma.
"It eats you up," Figley said. "It's a horrible thing. This is not unique. We've seen this in other wars."
Five soldiers who served with Cortez in Iraq testified that his actions were out of character and described the hardships of war they experienced, including sleep deprivation and the lack of running water.
"I just never would have seen it coming," said Staff Sgt. Tim Briggs, who has known Cortez for five years and served with him in Iraq.
Prosecutors said the stress was no excuse for the actions of Cortez and the other soldiers from the 101st Airborne Division, based at Fort Campbell.
On Wednesday, Cortez described raping the girl in her family's home in Mahmoudiya last March, along with Spc. James Barker, 24. Barker pleaded guilty in November to rape and murder and was sentenced to 90 years in military prison.
Barker has said in a sworn statement that the soldiers drank whiskey and played cards while plotting the assault.
Cortez said this week that former private Steven D. Green raped the girl before he did. Then Green shot her father, mother and sister before shooting the teen in the head, Cortez said.
He also testified that the soldiers tried to burn the girl's body. They burned their own clothes and threw the murder weapon, an AK-47, into a canal in an effort to dispose of the evidence.
Cortez was found not guilty of more serious charges of premeditated murder and conspiracy to premeditated murder.
Pfcs. Jesse Spielman, 22, and Bryan Howard, 19, await courts-martial. Green, who is accused of being the ringleader but was discharged from the military before being charged, will be prosecuted in a federal court in Kentucky.