The young Blues came from behind to claim all three points which sees them leapfrog the Reds at the top of the league.
Tahith Chong had put United ahead, but a brace from Keke Simmonds and one each from the centre-half pairing of Colin Rosler and Eric Garcia handed City a comfortable win.
It was United who struck first as they took the lead with the game only five minutes old.
Nishan Burkart used his strength to hold off Rosler on the right and when he played a pass across the face of goal, Chong was there to crash the ball into the top of the net from five yards.
City hit back immediately.
Henry Ogunby picked out Simmonds in the six-yard box and he showed his predatory instincts to equalise from close range.
The forward fired wide moments later after a good ball from Nabil Touaizi and then it was United’s turn in what was proving to be an end to end game.
Burkart’s pace took him away from Rosler, but Curtis Anderson stuck out a left boot to deny the striker from six-yards.
Lee O’Connor might have forced a second for the visitors, but unmarked, he headed wide of the post.
At the other end, Rosler made no mistake to give City the lead on the half hour mark.
Set up by another with City in his blood, Tommy Doyle, the centre-half showed great determination to get across his man and power a header past Alex Fojticek in the United goal.
City were growing in the ascendancy and Eric Garcia went close with a header before Felix Nmecha fired straight at the keeper, but the Reds were handed a lifeline via the penalty spot after Jeremie Frimpong felled Chong.
Skipper Angel Gomes stepped up but blazed his effort over the bar.
Gareth Taylor’s men extended their lead five minutes before the break, with Simmonds grabbing his second of the game.
Dancing inside off the left wing, he carved a yard of space for himself and fired a low effort into the bottom left corner from 20 yards.
The game became a more sedate affair in the second period, with neither side able to find that killer pass which resulted in a plethora of first half chances.
Both sides had spells of possession and the midfield duo of Iker Pozo and Doyle used the ball effectively as the young Blues patiently looked for a fourth goal.
As City did in the first half, United grew into the game and dominated possession for 10 minutes after the hour, but they struggled to create and when they did threaten the area, Garcia and Rosler were resolute in their defending.
Given the dearth of chances it was no surprise that when a goal did come it was via a set piece and it was the Blues who capitalised.
Substitute Ian Carlo Poveda delivered an in-swinging centre and Garcia was there to meet it as he sent his header into the top right corner.
Speculative efforts from distance courtesy of Fisayo Dele-Bashiru and Gomes for the away side failed to trouble either ‘keeper in the final 10 minutes as City saw out the game for a comfortable victory.
How they lined up
Gareth Taylor made five changes to the team that started against Stoke, with Anderson, Rosler, Ryan Corrigan, Doyle and coming into the side.
Anderson was in goal and protected by a back four of Jeremie Frimpong, Garcia, Rosler and Corrigan.
Pozo, Doyle and Nmecha were in midfield, while Ogunby and Simmonds flanked Touaizi.
What it means
City started the day in second place in the U18 Premier League but this win sees them leapfrog the Reds at the top of the table.
With seven wins and one defeat, the Blues have 21 points from the opening eight games and are one point ahead of second and third placed Everton and United, but with two games in hand on both teams.
There is a quick turn around for the young Blues who face a trip to Liverpool on Tuesday 28 November, while the U18 Premier League Cup campaign resumes next weekend away at Norwich City.