Blocking out intense speculation around his own future, United manager Erik ten Hag outwitted Pep Guardiola with a thought-out display against an off-colour City, as the Red Devils bagged themselves a spot in next season's Europa League.
Garnacho pounced on a defensive error to find an open net after half an hour, before Mainoo finished off a sweeping move to put their noisy neighbours on the ropes. Jeremy Doku's late strike squeezed past Andre Onana to set up a tense final 10 minutes, but United held on, and Ten Hag was lifted off his feet by Lisandro Martinez as the full-time whistle confirmed his second, and potentially last, trophy with the Red Devils.
Here, Express Sport takes you through four key talking points from an enthralling contest...
With intense speculation on his United future dominating the build-up to Saturday's game, Ten Hag will have been hoping for a statement performance. And that's exactly what he got.
Not only did the players give everything, but his tactics worked a treat as a solid bank of four comprising Scott McTominay, Bruno Fernandes, Marcus Rashford and Garnacho made it night on impossible for City to pass into midfield during the first half.
If reports that United bosses have already decided to part ways with Ten Hag are to be believed, the team's performance at Wembley suggests that the decision may have been a hasty one.
Our community members are treated to special offers, promotions, and adverts from us and our partners. You can check out at any time. Read our Privacy Policy
Alejandro Garnacho and Kobbie Mainoo were Man Utd's heroes at Wembley (Image: Getty)
United seal European spot
The Red Devils missed out on all forms of European qualification by finishing eighth in the Premier League table - their worst result in 34 years. But victory in the FA Cup sends them straight into the Europa League, boosting their pulling power in the transfer market, demoting sixth-place Chelsea down to the Europa Conference League and leaving Newcastle without any continental place whatsoever.
Ortega audition flops
City goalkeeper Ederson has missed the end of the season due to a fractured eye socket. And with rumours suggesting he could leave the Etihad Stadium this summer, back-up stopper Stefan Ortega may have been licking his lips at the prospect of a long-term starting role.
However, his clumsy showing at Wembley will have done him no favours on that front. After a handful a close calls while passing out from the back in the first period, Ortega's mix-up with Josko Gvardiol gifted Garnacho United's opener and set the tone for the rest of the half.
Invalid email
We use your sign-up to provide content in ways you've consented to and to improve our understanding of you. This may include adverts from us and 3rd parties based on our understanding. You can unsubscribe at any time. Read our Privacy Policy
Two academy graduates scoring United's goals in an FA Cup final sent the feel-good factor soaring in the red half of Wembley. Garnacho slid on his knees in front of despairing City supporters after netting the opener, and Mainoo was punching the air at the sky blue end less than 10 minutes later.
It's a momentous achievement for a pair of 19-year-olds to decide a game of such magnitude, and they each became the first teenagers to score in an FA Cup final since a certain Cristiano Ronaldo, sporting the same red shirt, two decades earlier.
Rodri run comes to an end
The midfield anchor has been one of City's most influential players this season. Generally speaking; when he plays, City avoid defeat. That has been the case for the past 74 games, or 475 days. Saturday's result will therefore give United even greater satisfaction having brought Rodri's staggering run to an end.