Ferrari and Leclerc blindsided the leading McLaren drivers with a brave one-stop strategy on an afternoon few teams thought a single tyre change could reach the chequered flag.
The Monegasque’s race was underpinned by a fast start that moved him up to third into the first corner, putting in position to capitalise on the warring McLaren drivers who led from the front row.
Norris had started from pole but found himself immediately under fire from Piastri. He swept right off the line to try to cover the sister car, but that served only to vacate the racing line for the Australian, who duly took it up.
He used the racing line to run side by side with Norris through Curva Grande up to the Roggia chicane, where a bold late-braking move swept him into the lead around his teammate’s outside.
It took the Briton by surprise, costing him momentum that allowed Leclerc to pass him with a better exit from the complex.
The Ferrari, freshly upgraded for its home race, was too quick to pass down Monza’s long straights, and with Norris gradually losing touch with Piastri, McLaren pulled the trigger on an early first pit stop on lap 14.
Leclerc followed him on the following lap, though the Ferrari driver lost position. Piastri held the lead when he made his own swap for fresh tyres one lap later.
It had looked like a decisive move for McLaren at the time, with Norris easily gapping Leclerc to pursue his teammate in what was expected to be the battle for victory. He and Piastri set an electrifying pace up front, trading new fastest laps between the, to underline the McLaren’s car superiority.
But Ferrari had already committed to a different tack. Rather than chase the leaders, Leclerc moderated his pace to ensure his tyres could make it the 38 laps to the finish.
By the time McLaren realised, it was too late to do anything about it.
Norris had already made his second stop - again early, on lap 32, after locking up in his escalating battle with Piastri - leaving him out of victory contention.
On lap 38 the team radioed the leading Australian asking whether he thought he could get to the end of the race without stopping again, the realisation dawning that the race was slipping away. Piastri, having burnt his tyres fending off Norris, replied that it would be impossible, and he was hurried into pit lane that lap for a fresh set of tyres.
The challenge stood at 18.5 seconds with 13 laps to go. Carlos Sainz, who was also one-stopping to the finish, defended admirably to hold Piastri up, delaying his rise into second place until lap 45.
With eight laps remaining the margin stood at a still significant 11.5 seconds. Piastri thrashed his car to the finish, but it was too little, too late.
Leclerc took the chequered flag with 2.6 seconds still in hand to claim send the packed grandstands in raptures over a shock Ferrari victory.
“Today I did not expect it going into the race,” he said. “I thought that we would struggle a bit more compared to the McLaren.
“To stand up on that podium in front of all the tifosi after such a week… to be able to win here is extremely special.”
The victory brings Ferrari to within 39 points of the constructors championship lead, revitalising the team’s season.
Piastri lamented that he and his team hadn’t spotted Ferrari’s tactics earlier but said he didn’t think the McLaren car would have been capable of matching the one-stop strategy.
“It was a big risk to do that,” he said. “That’s kind of the blessing and the curse of leading the race being at the front - the guys behind you can react to what you do.
“Charles, if he did a two-stop, he would have locked in third, and if he did a one-stop and fell off the cliff, he still would have finished third. But of course he pulled the one-stop off and Ferrari look like the hero today.
“Obviously it hurts at the moment, but I think in the moment it was the right thing to do.”
Norris, who extended his unenviable 100 per cent streak of failing to lead the first lap after starting a race from pole, was similarly downcast after seeing a golden chance to score big points against Max Verstappen for the drivers championship slip through his fingers.
“It’s disappointing starting from pole and ending third, especially when I think the pace was very strong,” he said. “A shame, but that’s what it is.
“We had a good qualifying car. But today our race car was probably not quite good enough.”
The two-three finish nonetheless closed McLaren to just eight points of the teams title lead behind Red Bull Racing, keeping it comfortably on track to take the lead before the end of the month.
Norris’s deficit to Verstappen shrunk to 62 points.
Sainz finished fourth in a strong race in service to his team’s ambitious strategy, while Lewis Hamilton was a lonely fifth as the lead Mercedes.
Verstappen, one year on from at this track breaking the record for most consecutive victories in F1 history, finished an anonymous sixth and a stunning 37.9 seconds off the lead.
The Dutchman started seventh and never showed competitive pace at a track that on paper should have suited his RB20 car, which the reigning champion said made him pessimistic that he and the team will be able to defend either title this year.
George Russell finished seventh after running off the road on the first lap and breaking his front wing, but the Mercedes driver still recovered to beat Sergio Pérez as the last of the frontrunning cars.
Thai driver Alex Albon drove an excellent one-stop race to head the midfield in ninth to score two points ahead of Kevin Magnussen, who finished ahead on the road but was demoted to 10th with a 10-second penalty for causing a collision with Pierre Gasly.
The stewards also slapped him with two penalty points on his superlicence, bringing him to a total of 12 for the year and earning him an automatic race ban for the next grand prix in Azerbaijan.
Fernando Alonso headed rookie Franco Colapinto in an in impressive debut grand prix performance. Daniel Ricciardo finished 13th ahead of Esteban Ocon, Pierre Gasly, Valtteri Bottas, Nico Hülkenberg, Zhou Guanyu and Lance Stroll.
Yuki Tsunoda was the race’s only retirement, calling it quits after being punted early in the race by an errant Hülkenberg.