Staff Sgt. Johnny M. Horne Jr., 30, of Winston-Salem, N.C., also received a reduction in rank to private, forfeiture of wages and a dishonorable discharge.
Horne pleaded guilty on Friday to one count of unpremeditated murder and one count of soliciting another soldier to commit unpremeditated murder.
Horne, attached to Company C, 1st Battalion, 41st Infantry Regiment, based in Fort Riley, Kansas, pleaded guilty to one count of unpremeditated murder and one count of soliciting another soldier to commit unpremeditated murder.
The charges relate to the Aug. 18 killing of a 16-year-old Iraqi male found in a burning truck with severe abdominal wounds sustained during clashes in Baghdad's Sadr City, an impoverished neighborhood that was the scene of fierce fighting between U.S. forces and Shiite rebels loyal to anti-U.S. cleric Muqtada al-Sadr.
A criminal investigator had said during an earlier hearing that the soldiers decided to kill him to "put him out of his misery."
A jury-like panel of seven service members late Friday sentenced after about four hours of deliberations, the military said on Saturday.
Horne was among six Fort Riley soldiers charged with killings in recent months — two for slayings in Kansas and four for deaths in Iraq. Staff Sgt. Cardenas J. Alban, 29, of Inglewood, Calif., is charged along with Horne in the teenager's killing and is awaiting a court-martial hearing.
Two other soldiers from the same unit this week faced Article 32 hearings — the military equivalent of a grand jury hearing — over a Sadr City killing in August.
Human rights groups have condemned the illegal killings of Iraqis — either civilians or wounded fighters — by the U.S. military, saying such acts amount to violations of international humanitarian rights and should be dealt with as war crimes.
Critics also say poor understanding by young U.S. troops of the rules of military engagement leads to the killing of civilians.
"It doesn't help you win the hearts and minds of the public if you put a bullet in their hearts and another in the minds," said Mark Garlasco, senior military analyst for Human Rights Watch.