Arnau Ortiz gave Atletico Madrid an early lead at the Strawberry Arena, curling a cross from outside the box into the far corner in the fifth minute after United failed to clear a corner. Michael Carrick's side dominated for long spells without finding a way through until the hour mark, when Shea Lacey won a penalty after being fouled by Jorge Castillo, and Mbeumo stepped up to convert.
The Cameroon forward then settled the contest in the 74th minute, tapping home after Jack Fletcher drove into the box from the right and squared the ball across goal, completing a fine comeback to win the Snapdragon Cup.
Speaking to MUTV afterwards, Mbeumo credited the team's preparation for the win.
"We had a clear plan before the game, everyone stuck to it and made the game enjoyable. We work really hard during training for these games, the main goal is to be ready for the season starting," he said.
Asked about scoring from different areas of the pitch having operated both as a winger and through the middle, he added: "I have done great things on the right but also as a striker. I think I am more experienced on the wing but I enjoy playing as a striker. For an offensive player, it is always good to score for the team."
Among the other talking points, 15-year-old JJ Gabriel — dubbed "Kid Messi" by some in the game for his exploits at academy level, with 20 goals in 22 appearances for Manchester United's Under-18s this season alone — got his first taste of senior football, coming on for the closing eight minutes.
Atletico's Jose Maria Gimenez wasted no time testing the teenager physically, catching him with a heavy challenge on his very first touch, though Gabriel was straight back on his feet and showed no lasting ill effects.
Nerves were understandable given the setting: a couple of passes from Mason Mount stuck under his feet before he cut inside for a blocked shot, but flashes of the composure that persuaded United to fast-track him past his own age group — he joined the club's academy at 11, the same summer Cristiano Ronaldo returned to Old Trafford — came through in a slick exchange with Leny Yoro and a run that won a foul in the box. Not bad for a player who will spend the rest of this summer preparing for his GCSE year rather than a full pre-season.
Amad Diallo, who signed a new long-term contract at Old Trafford earlier this year, arrived back from international duty under close watch after a modest domestic campaign that yielded just two goals from 32 Premier League appearances.
His displays for Ivory Coast told a different story: two goals at the World Cup and three more at the Africa Cup of Nations pointed to a player finding far better form on the international stage than he managed for his club last season. His first half against Atletico was a mixed bag: two of his opening dribbles were defensive interventions rather than attacking ones, and it was his attempted flick to Yoro that unintentionally set off the move leading to Ortiz's opener, with the resulting scramble ending in a corner and then the goal itself.
He recovered well before being replaced at the break, forcing a smart low save from Jan Oblak with a deflected free-kick and playing a role in the buildup that ended with Lacey's cross being headed wide by Mbeumo.
Elsewhere, goalkeeper Altay Bayindir looks set for an Old Trafford exit, with Celta Vigo having reached an agreement to take him on loan. Rather than featuring against Atletico, he spent the match working through a separate fitness session on the sidelines, a fate that had already befallen an earlier move to Besiktas, blocked back in January.
Karl Darlow's free transfer arrival from Leeds this summer effectively confirmed Bayindir would no longer be needed as back-up to Senne Lammens, with the 28-year-old having made 17 first-team appearances since joining from Fenerbahce.