First game of 2015 sees young U's win at Cardiff.
The match was level pegging in the first half, with Byron Lawrence and Macauley Bonne both having opportunities to break the deadlock in the opening forty-five minutes.
But it was two U's substitutes who made the difference late on, on a positive night for the club in South Wales.
Conor Hubble scored the first, slotting home to give the U's the lead on 83 minutes.
Danny Johnson nearly equalised straight away for the home side, before the U's doubled their advantage.
City keeper Ben Wilson made a good save from Dion Sembie-Ferris, who had made his first team debut at the same stadium on Friday night, but Nnamdi Nwachuku was on hand to tuck home the rebound.
The U21s next home game is against Crystal Palace on Monday January 12th at WHCS, 7pm kick-off.