Ronaldo scored twice in the second half, goals number 800 and 801 of his incredible career, to secure a priceless 3-2 victory for the Red Devils.
His first came in the 52nd minute, the former Real Madrid man sweeping the ball home from Marcus Rashford's pinpoint cross to make it 2-1.
Arsenal responded shortly after through Martin Odegaard, who then clumsily gave away a penalty in the 70th-minute to hand Ronaldo the chance to put United back in the lead.
And he was never, EVER, going to miss from 12 yards.
Video: Ronaldo's match-winning penalty v Arsenal
Ronaldo fired the ball past a despairing Aaron Ramsdale in the Arsenal goal, making it 12 goals in 16 games since returning to Old Trafford.
The 36-year-old is just an incredible footballer and in the 87th-minute, Ronaldo reminded the world that he's still willing to put his body on the line to help the team.
Despite the fact he was clearly struggling with injury, the five-time Ballon d'Or winner sprinted as hard as he could to join in with a United counterattack after he'd cleared an Arsenal corner himself.
Ronaldo brace sinks Arsenal | Manchester United 3-2 Arsenal | Football Terrace
Unfortunately, the ball didn't arrive at his feet in the opposition's box, with Bruno Fernandes instead opting to shoot without reward rather than pass to his Portuguese colleague.
But take nothing away from Ronaldo's unwavering desire to score and you can check out the latest example of the global icon's elite mentality below.
Video: Ronaldo sprints to join 87th-min counter while injured
Almost straight after he'd busted a gut to get into the penalty area, Ronaldo was replaced by Anthony Martial and the match-winner worryingly trotted off down the tunnel rather than sitting on the bench.
United fans will be praying that the injury is not serious and that the team's main man can feature in Ralf Rangnick's first match in charge versus Crystal Palace on the weekend.