Having conceded inside 13 seconds in last year’s showpiece, United avoided a similar fate at a sun-drenched Wembley, with Garnacho seeing a sharp early effort thwarted by Stefan Ortega.
Pep Guardiola’s side slowly began to take control of proceedings after a disjointed start, but it was the Red Devils who broke the deadlock shortly before the half-hour mark as Garnacho capitalised on a calamitous mix-up between Josko Gvardiol and Ortega to tap into an empty net.
Armed with the momentum, United continued to grow in confidence as the first half progressed and Ten Hag’s men duly doubled their lead in the 39th minute.
There might have been a touch of fortune about the opener, but United were at their free-flowing best to score the second, as a brilliant team move involving Marcus Rashford and Garnacho culminated in Bruno Fernandes laying the ball off to Mainoo, who stroked a clinical finish into the far corner.
Faced with a two-goal deficit, Guardiola introduced Jeremy Doku at the start of the second half in the hope of injecting a spark into City’s attacking play.
That substitution almost had an immediate effect with the winger finding Erling Haaland free in the box, only for the Norwegian to crash his strike against the crossbar.
Undeterred by that miss, City continued to search for a route back into the contest, but Andre Onana stood firm in the United goal to deny Kyle Walker from range before Julian Alvarez fluffed his lines with a glorious one-on-one opportunity.
The City pressure showed no signs of slowing down with time ticking into the final 15 minutes, and Guardiola’s side finally pulled a goal back when Doku’s effort from the edge of the box beat Onana at his near post.
That set up a fascinating finale but despite a late onslaught from City, Ten Hag’s men held firm under pressure to secure a memorable triumph.
The result gives United a first FA Cup trophy since 2015/16 and a much-needed boost after their lowest-ever Premier League finish, while City miss out on the chance to become the first English side to win back-to-back league and cup doubles.
Flashscore Man of the Match: Sofyan Amrabat (Manchester United)