The 19-year-old Mercedes ace Antonelli took pole by just 0.043sec on Saturday, powering around the famed street circuit to edge out two all-time greats, Max Verstappen (1min 12.094sec) in his Red Bull, and Ferrari's Lewis Hamilton (1:12.279).
Antonelli saved his inspired best for last to ensure Mercedes have taken pole for every grand prix this season, and the young championship leader couldn't stop beaming.
"It was one of those laps that we call a magic lap. I was able to put it all together," he smiled, having put on a particularly notable display considering teammate and title rival George Russell could only finish sixth.
While Verstappen was buoyed by his Red Bull's performance, his teammate Isack Hadjar backed him up by finishing fifth quickest.
But it's back to the drawing board for McLaren, who'd been hoping for a special performance on their landmark weekend.
Instead, Piastri was seventh, his 1:12.624 lap being more than half-a-second down on Antonelli's 1:12.051, with his world champion teammate Lando Norris eighth (1:12.765).
On a circuit where it's almost impossible to overtake, that spells a potentially long fruitless afternoon for the McLaren pair on Sunday, with reigning Monaco champ Norris admitting with a rueful smile that he was "already looking forward to Barcelona" for next week's race.
Piastri, who had a brush with the wall on the tight circuit in the second part of qualifying, was unsurprised by McLaren's disappointing day, especially after finishing over a second slower than their main rivals in Friday's practice.
"It's been a long time since we've been a second off on genuine pace," he shrugged. "So yesterday was tough. I think we made the car better, and the balance was nicer today, but it was still half-a-second off.
"While we've made some progress since Friday and improved the balance of the car, we're still lacking the speed.
"Looking ahead to the race, we have to be realistic. This circuit is notoriously difficult for overtaking, and we don't expect that to change."
Antonelli, who'd earlier on Saturday signalled his intentions by finishing quickest in third practice after a disappointing Friday session, will now be a warm favourite for a fifth grand prix triumph in a row on Sunday.
That's a feat only Verstappen and Hamilton have achieved among current drivers, while the Italian is in a strong position to extend his championship lead of 43 points over teammate Russell.
Verstappen, after a season of moans about F1's new look, sounded much more chipper, noting of his drive that's got him on the front row: "Once you get on top of it and get a clean lap out of it, especially in qualifying you're flat-out and on the limit, it's very rewarding when it goes well."
Ferrari will be disappointed to have missed what felt like a golden opportunity, with Hamilton third and local hero Charles Leclerc fourth in 1:12.351, although the Monegasque's car needed some repairs after he clipped a barrier on his final run.
Hamilton, whose girlfriend Kim Kardashian made her first appearance in the F1 paddock on Saturday to add more glamour to the glitzy Monaco circus, still felt he was in line to complete the "impossible" dream by earning his maiden Ferrari victory and the 106th win of his career.
"I probably need rain, but nothing's impossible," he shrugged. "I'll try to get in there and hassle these two (Antonelli and Verstappen) as much as I can and force them into not making certain corners."
