A 60-year-old woman thought she was going to die when a drunk US Marine with a sensitivity to alcohol launched an unprovoked assault upon her, repeatedly punching and smashing her head.

Lance Corporal Taylor Wyatt Elwood, 20, has been jailed after pleading guilty to assault after dragging the woman from her car at Brisbane's Enoggera barracks car park in July.

He also assaulted an off-duty Australian Federal Police officer who came to her aid.

Elwood will serve three months in jail after being sentenced in Brisbane Magistrates Court on Friday.

US Marine Taylor Wyatt Elwood has been jailed for the drunken assault of a woman in Brisbane. (AAP)

He will also be dishonourably discharged and possibly court-martialled by the Marines after completing his punishment in Australia.

Elwood was drunk when he donned a motorcycle helmet and al- black clothing before suddenly attacking the woman, a barracks staffer, unprovoked about 3.30pm on July 3.

As he punched her head and stomach, she screamed for help as loudly as she could.

She was also thrown against a guard rail and cars.

"(He seemed) obsessed with her head and kept banging it and punching it over and over," police prosecutor Glenn Whittle said.

Elwood told his victim "just be quiet" in a tone she described as having "no emotion" and in a "normal voice", the court heard.

The woman felt she had no control of her body but "willed herself not to black out", according to the prosecution.

Supporters of Lance Corporal Elwood leaving court today. (AAP)

She described the attack as a "well rehearsed move or a dance" and was hospitalised with bruises.

"She felt like a rag doll being thrown around. She believed she was going to die," Sergeant Whittle said.

When two men stopped after seeing Elwood standing over her, he grabbed, punched and bit one of them – who was an AFP officer.

Elwood's defence solicitor Dave Garratt said he had a high sensitivity to alcohol.

The promising high school American football player, who chose the Marines over a college scholarship, had seen his career go "up in flames" over the incident, Mr Garratt said.

With Elwood's unit completing an exchange at a Darwin barracks, he had been under house arrest, the court heard.

Magistrate Tina Previtera sentenced him to 15 months in jail, which will be suspended after three months.

He will also pay $3000 restitution to the woman.