US Master Sergeant John E. Hatley had also been found guilty Wednesday of conspiracy to kill the unidentified detainees, but was acquitted of a fifth charge of premeditated murder and of obstruction of justice.
Hatley, who prosecutors said masterminded the killings he committed with two other soldiers, will begin serving his sentence in Germany before being transferred to a US prison. He will be eligible for parole in 20 years.
During the sentencing hearing, the 40-year-old Hatley told an eight-man army panel that he respected their findings but recounted the stress of dealing with mounting US casualties at the hands of insurgents.
"I understand your decision," said Hatley, who had pleaded not guilty. "I'm not perfect, I ain't no angel" the sergeant said, fighting back tears as he spoke of "policing (cleaning) up the pieces of our soldiers, civilians and friends" following bomb and sniper attacks.
Hatley estimated his company of 150 men had engaged in some 130 firefights, while an army chaplain said they had also suffered a "nightmare" of "easily 250" roadside bomb attacks. "We knew people were going to die," Chaplain Seth George added.
But Hatley denied he had cracked under the pressure of relentless attacks, telling the judge "I'm not crazy, sir," and adding: "I'm an infantryman, sir."
Defence attorney David Court said Hatley had taken the sentence "stoically."
The defendant had expressed no emotion when the verdict was announced late Wednesday, but embraced his wife Kim and fellow soldiers and friends who had stood by him during the four-day trial.
Hatley had been accused of shooting prisoners in two separate incidents but was declared not guilty of the January 2007 death of a detainee who was already seriously wounded.
The second shooting - of four blindfolded suspected insurgents - took place in late March 2007 in or near southwest Baghdad.
Hatley was the highest ranking of three soldiers tried for killing the prisoners, who were shot "execution style," according to army prosecutors. The bodies, which witnesses said were dumped into a canal, were never found.
The trial was held near the southeastern German town of Vilseck because Hatley's unit has redeployed to Germany.
One army prosecutor, Captain John Riesenberg, charged Thursday that Hatley had committed one of "the most colossal failures of leadership that you can conceive of."
Riesenberg slammed the "truly atrocious nature of this quadruple homicide" and said US troops must serve in "an army that holds itself to the highest standards."
But Court, the defence lawyer, told the panel that "John Hatley is not the evil person the government has portrayed" and said "every war has this problem" of soldiers reaching the "breaking point."
Private Michael Leahy, a combat medic, and Sergeant First Class Joseph P. Mayo had already been convicted here over the killings and sentenced to life and 35 years in prison respectively, with the possibility of parole.
Two other soldiers were convicted of lesser offenses, and charges against a final two were dismissed in return for their cooperation in the investigation. Leahy and Mayo testified the detainees were shot point-blank in the back of the head with pistols hours after they were captured near a cache of assault rifles, a duffel bag of bullets, grenades and two sniper rifles.
Mayo said the killings were meant to protect his troops against attacks that had already killed six of the unit's men.
Hatley, described by many, including an Iraqi interpreter, as an outstanding sergeant, cried when he told the court "I love those guys," and asked if it was possible to love his men "too much."
Court concluded by saying: "He did, he loved his men too much. But 144 of them came back (alive) because of that love."