An inexperienced United team had to wait until the second half to score, with substitute Marc Walton grabbing a brace.
The win gave a valuable boost to the reserves who have had a disappointing league season and 16-year-old match winner Walton was delighted by the score.
"That was the best feeling in the world," he told the Evening Chronicle.
"Scoring two goals at St James' Park was fantastic. It was the first time I'd played there.
"I'd been in the squad for the play-off game against Manchester United last year but hadn't been involved so I was hoping to come on and to score two goals was brilliant."
The reserves now round off their season by playing in a benefit game at Ashington on Friday, with the proceeds going to the family of hit-and-run victim Kirsty Sawyer.