Matheus Cunha scored a sensational 87th-minute winner to clinch a 3-2 victory for Michael Carrick's Manchester United and cut Arsenal's lead at the top of the Premier League to four points.
Substitute Cunha curled a stunning finish into the corner from 25 yards out, only three minutes after Mikel Merino had hauled the lacklustre hosts level from a Bukayo Saka corner.
Earlier, Arsenal had squandered a lead when Martin Zubimendi's poor pass allowed Bryan Mbeumo to run through on goal and beat David Raya after Lisandro Martinez had diverted Martin Odegaard's cross-shot into his own net for the opening goal.
Arsenal struggled to regain their composure after Mbeumo's leveller and Manchester United, 2-0 winners over Manchester City in Carrick's first game in charge last weekend, went in front when Patrick Dorgu crashed a superb effort in off the bar.
Mikel Arteta tried to change the flow of the game by making a quadruple substitution before the hour mark, but Arsenal continued to flounder before finally restoring parity when Merino prodded over the line for yet another set-piece goal.
At that point, it seemed Arsenal might go on and find a winner, but Cunha silenced the Emirates Stadium when his stunning, long-range effort sparked more wild Manchester United celebrations. There were boos from some home fans at the final whistle.
The victory continues a dream start in the dugout for head coach Carrick, and leaves alarm bells ringing for Arsenal, whose lead at the top of the Premier League is reduced from seven to four points following Manchester City and Aston Villa's wins.
Having been held to goalless draws against Nottingham Forest and Liverpool in their previous two Premier League games, Arsenal are now without a win in three as Manchester United continue to push for a Champions League finish in fourth.
Former Arsenal captain Patrick Vieira felt his former side showed a lack of mental strength.
"They are still four points clear," he said on Your Site. "But there are still questions about the mental strength of the team.
"It's not just that they lost the game, it's the way they lost the game. Saka and Trossard didn't produce enough to worry United.
"They need a leader to lift the spirit of the team. They need to understand when they are on the pitch they have to play with more energy and more risks. They have the quality.
"They didn't play with the freedom to express themselves."
Arsenal manager Mikel Arteta speaking to Your Site about reminding the fans where Arsenal are following boos at the Emirates Stadium.
"I hope we don't have to do that, because they know," he said.
"We need them when we don't win. When we win it's easy.
"When we lose a game we have to show togetherness, and unity, the energy to get where we want.
"Defeats are part of that for us and for any team.
"A very painful one but it's time to be close to the players who gave us great moments. We all want to win.
"We have to learn from it. Defeats are part of winning, and we have to take them like this, because they need to be reminded how good they are."
Arsenal captain Martin Odegaard speaking to Your Site:
"It wasn't good enough from us. We need some time to analyse it now but it was not good enough, the performance. We should have done better but now is the time to stick together even more and keep working, be together, and bounce back.
"In the first half, we were the better team, scored the goal and had control of the game but we had a lot of giveaways in the game that created dangerous moments and in the second half the game changed a little bit.
"We know the quality they have when they get the space to run. In the second half, there were a lot of duels they won that gave them the momentum of the game and let them run and we never really managed to get into that rhythm of creating chances.
"It was never going to be easy in this league, we know that. Now is the time to be strong. We are still top of the league so we have to keep going and bounce back straight away."
Man Utd head coach Michael Carrick speaking to Your Site:
"Massively proud. I think it was different from last week. It was always going to be different today. They put us under a lot of pressure in different ways.
"We probably didn't have as much genuine, regular threat within the game as we did last week, but I thought there were periods when we looked really dangerous.
"To come here and score three goals in the manner we did, going one down, coming back, then the setback, and still keeping going - it was a huge moment for us.
"The boys had to put an awful lot in today. It probably didn't come as easy as it did last week. Last week, the time, the flow of the game and the momentum was with us pretty much throughout, whereas today we had to dig deep.
"It was a massive effort and I'm really proud of them."
Man Utd defender Harry Maguire speaking to Your Site:
"It's absolutely massive. We know the first game under Michael Carrick was the Manchester derby at Old Trafford. You always get up for those games and results like that can happen. But we knew we had to back it up today.
"We spoke about it before the game, coming here to face the league leaders, who have been excellent this season. They ask a lot of questions of you, and to go and get the winner after a late setback shows the character of this team. It was a magnificent performance."
