Chido Obi had put the young Reds ahead shortly before half-time, after winning possession high up. An even game included spells of dominance for each side, and chances aplenty, with City levelling 15 minutes from time when Reigan Heskey poked neatly past Will Murdock from close range.
United showed resilience to wrestle back control of the fixture and lead once again, with substitutes Shea Lacey and Amir Ibragimov combining for a late winner.
United showed intent from kick-off, with Sam Mather’s neat footwork taking him past a couple of markers before a well-weighted pass teed up James Scanlon to shoot when cutting in on his left.