But as much as Sunday was Mainoo's, it was an even bigger triumph for the man who has turned the England midfielder's career around.
The job is Michael Carrick's. Indeed, the Geordie has been the one the board favoured barely weeks into this caretaker tenure. With the club's Champions League qualification assured, the debate now between the top brass and Carrick, himself, is over whether to announce his long-term (at least, longer term) contract immediately, or wait until the end of the season to do so.
Football Presse being told that there is some thought going into whether it would spur the players on by confirming Carrick with three games still to play - or the news would become a distraction. While third place and the final automatic Champions League qualification place appears assured, mathematically, United are still at risk of finishing fourth and dropping into the summer playoff places. As we say, the debate is now whether to leave things alone with a sense of momentum back amongst the players, or to give the team and support that extra lift by announcing Carrick's new contract.
Either way, the decision has been made. And for this column, it's the right one. Carrick has cajoled and inspired an inherited dressing room that was at rock bottom. Confidence? Belief? The gauge was on empty. The management "had no idea". The players were "the worst to fill a United shirt in a generation". The club was facing "more years in the wilderness".
That was barely four months ago. Today? Well, thanks to Carrick and his staff, Manchester United is a club and a team transformed. Mainoo, for one, is the perfect example of why Carrick deserves this new, long-term deal.
Ruben Amorim has been pilloried for his treatment of the England midfielder. And rightly so. But the poor management of Mainoo goes beyond the Portuguese. The 21 year-old, before Carrick's intervention, was virtually transfer-listed. The player's camp were actively hawking his name around the Premier League and across Europe - with the clear permission of United's top brass.
Rather than seek to negotiate a new deal with Mainoo, management were happier to co-operate with his agents and find a buyer. A player who had come right through the club's system. Capped at senior level at the age of 18. Yet, effectively written off by both the coaching staff and senior management. And beyond that, also by those ex-players on the outside. There was barely a whimper of protest from United-connected pundits as it was becoming more and more apparent that Mainoo would be moving on.
That is, until Carrick and his team arrived. Then everything shifted. Finally, United had someone in charge who understood the club. Who understands the value of a homespun player, especially one of Mainoo's quality, and what he offers on the pitch, around the club and to those in the stands. It's those intangibles that only someone who served Manchester United as Carrick has could not only understand, but argue successfully for.
Within days of Carrick's appointment, Mainoo was not only withdrawn from the transfer-list, but also thrown into the starting XI. Carrick, who had worked with the player during his previous coaching stint at United, was always there, encouraging, motivating - but he wasn't alone. Steve Holland, an inspired appointment by Carrick, has also been there to lift and rally the players - including Mainoo. The ability. The potential. It never left Mainoo. What had disappeared was the support he had enjoyed from coaching staff and those higher up. Again, it must be remembered, Amorim was at the pointy end of all this, but no-one upstairs intervened. No member of management urged the then United manager to take a second look at their young academy graduate. Nothing was going to change until Carrick stepped in.
And for Mainoo, you can swap his situation with any number of teammates. Bruno Fernandes, for one, is now being mentioned as a candidate for the Player of the Season awards. Yet, again, he was being pulled apart by pundits for how he handled the club's captaincy. And beyond that, like Mainoo, the higher-ups had been actively seeking to sell the Portugal international.
Casemiro. Diogo Dalot. Harry Maguire. The list goes on and on... Luke Shaw. Now all regarded as indispensable, but before Carrick, they were more out than in. Casemiro's departure was agreed over those first weeks in January amidst Carrick's arrival. If that decision had been delayed just a month, where would Casemiro and United be now? Those top brass, as much as Carrick has rescued their reputation - and indeed careers - in this game, their poor decision-making, in terms of the Brazilian veteran, will still cost the club.
You do wonder about Casemiro's plans if he'd just waited, with Carrick in charge, a few more weeks. But again, that announcement in January is simply another outpost of how big a transformation this club and team has undergone since Carrick's arrival.
Of course, Michael Carrick deserves this new, long-term contract. He's revived the team, he's rescued the club and fixed several damaged reputations. Indeed, Carrick doesn't just deserve this Manchester United contract, those making the decisions bloody well owe it to him.