Sergeant Selena M Salcedo, 24, pleaded guilty on Thursday to dereliction of duty and assault, admitting she kicked the prisoner, grabbed his head and forced him against a wall several times.
"I just want to apologise to my mother for making her go through this; to you for being in your court," she tearfully told a military judge. "I'm a better soldier than that."
Military prosecutors agreed to drop two related charges. She was reduced in rank to corporal or specialist, given a letter of reprimand and docked $250 a month in pay for four months.
She could have got a year in prison and a bad-conduct discharge. Salcedo admitted mistreating a prisoner known as Dilawar, whom the military was holding at Bagram air base in Afghanistan in 2002.
She told Lieutenant-Colonel Mark Sposato, the military judge, that she knew what she was doing was illegal but became frustrated when Dilawar avoided her questions.
"I got frustrated so I grabbed the side of his face to include his ears" and pushed him against a wall, Salcedo said.
"I'm a better soldier than that"
Selena M Salcedo,
US Army sergeant
She said she repeatedly forced Dilawar to sit against the wall as though he were in a chair. Salcedo said Dilawar complained of knee pain, but she believed he was just trying to avoid further questioning.
Dilawar was apparently also beaten by others, and several other soldiers have been charged. Among them is Private Willie V Brand, an Army Reservist accused of beating Dilawar over a five-day period in 2002 at Bagram.
Dilawar died shortly afterwards. An autopsy showed that his legs were so damaged that amputation would have been necessary had he survived.
Murder charges against Brand have been dropped, although allegations of abuse and other charges still stand. A general court martial for him is scheduled for 15 August.