It was, quite simply, the finest Manchester United result since the retirement of Sir Alex Ferguson in 2013. The bar has been lowered, the trophies drying up but this was a day for their fans to celebrate wildly – and not only because they stopped Manchester City from making yet more history.

Another Premier League and FA Cup Double had been on for Pep Guardiola and his all-conquering team to follow the one from last season and nobody gave United a prayer. The bookmakers had them at 8-1 to win. Erik ten Hag’s time as manager is up after the most traumatic of seasons; eighth in the league their lowest finish since 1989‑90, chaos at virtually every turn. And yet United refused to yield.

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It was the young stars, Alejandro Garnacho and Kobbie Mainoo, who scored their goals during a golden first half; never before have two teenagers scored in an FA Cup final. But this was a triumph of the collective resolve, exemplified by the central defenders, Raphaël Varane and Lisandro Martínez. And the captain, Bruno Fernandes, whose sumptuous pass for Mainoo’s goal will live long in the memory. It was the first time since 1 February Ten Hag was able to start Varane and Martínez together; what a difference they made.

City laboured uncharacteristically in the first half but it was a different story after the interval when they were dominant, creating plenty only to lack the killer touch. It looked as though United had plotted a course through the storm. And yet their addiction to drama saw André Onana throw City a lifeline, the goalkeeper caught out at his near post on a low long-ranger from the substitute Jérémy Doku.

It was a horrible moment for Onana and it was nerve-shredding for everyone connected to United as the board showed seven additional minutes. But United did not let it slip and when the final whistle blew it was possible in those moments of delirium to forget – however temporarily – much of what had gone before. This stood in glorious isolation, the end of a wild cup run that had been ignited by the epic quarter-final win over Liverpool. There would also be the bonus of Europa League qualification.

City did not do enough, not a sentence that has been written too often in recent times. They hogged 75% of possession but they could not pull United apart with the regularity they wanted. When they had their chances in the second period, they were profligate.

City had scored inside 13 seconds through Ilkay Gündogan in this showpiece against United last season – step two of their march towards the Treble – but here they sought to draw their rivals’ sting with patient possession.

United’s concentration, though, was on point and their supporters were in dreamland by the interval; two goals to the good and with Onana having not been threatened.

Fernandes’s assist was the highlight. He had seen the picture as Garnacho drove in from the right and when the ball came to him, he diverted it with the deftest of touches for Mainoo, who had run off Kyle Walker. Mainoo was progressive and courageous in possession throughout. He finished with breathtaking assurance.

United brought the hustle, at times, in the first half as City tried to play out from the back and the breakthrough goal had been all about the persistence of Garnacho, who harried Josko Gvardiol as they chased a high Diogo Dalot punt forward.

Cue a disastrous mix-up between the City defender and Stefan Ortega. After the ball had bounced, Gvardiol went to head back to Ortega except the goalkeeper was not there; he was out of position. Garnacho reacted the quickest to run on to the loose ball and tap into the empty net.

Related: Erik ten Hag reminds Manchester United of what could have been | Jamie Jackson

City’s reset at half-time was comprehensive, Guardiola’s big move being Doku for Mateo Kovacic with Phil Foden moving into central midfield on a permanent basis.

Previously, he had roamed from a starting position on the left. Ten Hag’s big tactical move at the outset had been to continue without a pure No 9, Rasmus Højlund among the substitutes. It was Fernandes as the false nine; Scott McTominay in support.

City pushed and it was clear United would live or die by their defensive work. Doku crossed low for Foden but City’s main man could not sort out his feet. From another Doku cross, Erling Haaland spun and rattled the crossbar. Walker also detonated a long-range blast Onana did well to turn around the post.

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Foden was at the heart of everything, his status further enhanced when Guardiola hooked the out-of-sorts Kevin De Bruyne and introduced Julián Álvarez. United wanted to chalk off blocks of time. And City misses. Álvarez provided two more of those, lifting one chance high and dragging another wide of the far post after a Foden pass. He had to do better.

United struggled to get out of their half in the second period but it was only about keeping City out, the mission complicated when Martínez succumbed to cramp. On came Jonny Evans. What did City have left? The Doku goal that hinted at redemption. And nothing more.