Mainoo closed down Kevin De Bruyne and headed on a loose ball out to Bruno Fernandes before racing away on the attack.
He found himself on the end of a superb Manchester United move, side-footing an effort past Stefan Ortega after Fernandes' smart flick across the box.
Including the brilliant drilled pass from Marcus Rashford and the directness of Alejandro Garnacho, it was the type of front-to-back goal Erik ten Hag would have dreamed of putting together pre-game.
And it was poignant that Stockport-born United fan Mainoo was there to convert in front of a visibly shocked Manchester City half of Wembley Stadium.
"Last year I was in the stands," Mainoo said at full-time, taking in the atmosphere at pitch level.
"(Winning the FA Cup means) absolutely everything. It's been a tough season with ups and downs and this is the only thing we've had to look forward to.
"We knew we had to come together. The preparation for this game has been amazing. We've shown we can compete and win games."
But his goal was a bonus - the cherry on top of an energetic but composed FA Cup final display against some of the world's best in midfield.
Paired with Sofyan Amrabat in a deeper role, Mainoo was so effective in helping to disrupt and ultimately suppress City's main threats.
The 19-year-old more than held his own against Rodri and ate up the spaces the Bernardo Silva, Phil Foden and De Bruyne often like to take up.
His super work against De Bruyne in particular was vindicated when Pep Guardiola substituted the Belgian before an hour had even been played.
In possession, Mainoo was just as good - playing neat and simply as United cut through City's midfield beautifully at various points.
But he wasn't one to just sit and admire his intricate bits of play, instead wanting to get forward and have an impact the other way too.
His intent was summarised as the clock ticked down when he triggered a counter-attack in the final 10 minutes led by Rasmus Hojlund, who had a very useful cameo off the bench.
United sensed they could truly finish off their rivals as a handful of players piled forward and Mainoo - still looking fresh in the dying embers - was one of them.
Referee Andy Madley's full-time whistle dropped Mainoo to his knees, allowing him to take in and process what he and his teammates had achieved.
Mainoo took in the celebrations with his newly-acquired FA Cup winner's medal, donning a United bucket hat and embracing his ever-growing set of fans.
And what they all witnessed on Saturday was one of their own living every United supporter's dream.
