The young Blues ran out 3-2 winners over the Reds, who pipped them to the League title and beat them 3-1 at Old Trafford in the final league game on April 28, in front of a crowd of 4,783 at COMS.
United’s Giuseppe Rossi cancelled out Ian Bennett’s opener and David Bellion struck on the stroke of half-time to put United in front.
However, City came back in the second half with the superb Ishmael Miller the catalyst.
He came on in the 68th minute with City still trailing 2-1 but made Stephen Ireland’s equaliser before netting the winner with 4 minutes remaining.
Both keepers were forced into early saves, Nicky Weaver denying Rossi while Tom Heaton prevented Bradley Wright-Phillips from scoring.
Wright-Phillips and Colin Heath went close for each side before Bennett gave City a 33rd-minute lead with his first goal of the season, which came from Lee Croft’s right-wing cross.
United levelled 2 minutes later though, with Rossi converting from the spot after Danny Warrender was adjudged to have blocked Lee Lawrence, and Bellion gave the visitors the lead in first-half stoppage time.
City were denied by the bar on two occasions in the second half, after Heaton saved Croft’s fiercely-struck shot and Micah Richards headed Ireland’s cross, and it looked as though City would be Senior Cup runners-up for the 4th straight season.
But Miller was then introduced with an immediate effect, his left-wing cross from the byline being forced home at the near post by Ireland.
Miller then won the tie with a great effort. He teased the United defence on the edge of the box before curling the ball into the top-right corner.
The game wasn’t quite won yet though, with Weaver having to pull off a superb stop to prevent Spaniard Gerard Pique from sending the tie to penalties.
City: Weaver, Warrender, Collins, J D’Laryea, Richards, Bennett (Miller 68), Croft, Flood, B Wright-Phillips (Laird 89), Bermingham, Ireland.
Subs: Schmeichel (GK), Lee-Matthews, Grimes.
United: Heaton, Picken, Martin (Bardsley 46), Pique, McShane, Fox, Bellion, Ngalula, Heath (Lawrence 24), Rossi, Eagles (R Jones 76).
Subs: Lee (GK), Calliste, Neumayr, Hogg, Eckersley.