Leclerc climbed to the top of the times in the final third of the hour-long FP1 to eclipse his main championship rival by a slender 0.09 seconds, while Carlos Sainz completed the top three.
George Russell, meanwhile, was fourth in the Mercedes W13, which debuts a revised nose cooling duct this weekend as track temperatures soared towards 60 degrees Celsius.
Seven-time champion team-mate Lewis Hamilton, however, watched the session alongside team boss Toto Wolff from the pit garage to make way for F1 debutant Nyck de Vries.
Verstappen comfortably set the pace during the opening round of fast laps, the Red Bull driver sitting half a second behind his team-mate thanks to a benchmark effort of 1m34.991s.
It had been Sainz who led Robert Kubica, replacing Valtteri Bottas in the Alfa Romeo, and Zhou Guanyu at the circuit with the Ferrari driver who initially set a lap of 1m39.577s.
But that was quickened by Kevin Magnussen and then Lando Norris, who set a 1m37.604s despite complaining of a spongy feel from the accelerator pedal.
Leclerc improved to 1m37.420s before Verstappen, staying fit for 2022, blew the bankers away at his first attempt as he ran 1m35.727s to find 1.6s over the Ferrari.
Perez drifted away by around 0.4s, both RB18s on a pair of soft redwall C4 tyres, while the updated McLaren MCL36 enjoyed a 3-4 with Daniel Ricciardo leading Norris.
Verstappen then lapped to his 1m34.991 to end the opener, initially extracting 1.1s over Perez to allow an improved Leclerc to quickly split the Red Bull pair.
Leclerc, who is 38 points behind Verstappen, was immediately relegated by Pérez, the Monaco GP winner who responded to the Scuderia’s threat by closing to within 0.5s of his stablemate.
But the Mexican would soon suffer a high-speed spin at Turn 3 when he collected too much inside the curb to unsettle the car, locking up all four tires as he slowed to miss the wall.
After a mid-session lull, with 30 minutes to run, Verstappen emerged with a brilliant new set of soft Pirellis to reduce the times to 1m34.346s thanks to a particularly strong first sector.
But Ferrari soon found an answer as Sainz touched the line in 1m34.268s to find himself eight hundredths clear of his Red Bull rival, the F1-75 doing its heavy lifting in the final sector.
Verstappen responded with 21 minutes to go, setting a 1m34.021, but left time on the table going over the edge of the exit of Turn 12 after understeer on the approach.
The Dutch ace would also notice slow gear changes, although this appeared to be related to him clattering the pavements of the turn 8-9 chicane.
Those brief moments left the door open for Leclerc to clock a 1mm33.930s, the only driver to turn under 1m34s.
With the riders stuck to the now-scrubbed Pirellis for the remainder of the hour, the Monegasque maintained his 0.09s advantage over Verstappen to claim early bragging rights.
Sainz finished 0.3 seconds behind his team-mate and will now face at least a 10-place grid penalty after taking out his third control electronics package of the season.
Breaking the upper limit of two as a legacy of his catastrophic engine fire at the Austrian GP, like Leclerc in Montreal, further success could come if Ferrari changes more engine parts as the event progresses.
Russell, meanwhile, was the best of the rest in fourth. The Brit ran 0.9s adrift with the W13 noticeably lifting its inside wheel as a result of the stiff setup to combat the porpoise.
Pierre Gasly made good use of AlphaTauri’s first significant improvement of the campaign to finish fifth and overtake Perez.
It continued to be a messy session for the Mexican, who lined the chicane and had to stop briefly in the Red Bull garage after his visor became caught in the right rear suspension.
The RB18 also features modifications for Paul Ricard, thanks to a revised floor, with Adrian Newey getting down on his knees to inspect Verstappen’s boat late on.
Norris ran the tuned McLaren (notable for its new sidepods) to seventh ahead of Albon’s Williams, while Formula 2 and Formula E champion de Vries finished ninth, having made a good lead in principle to risk damaging the floor.
Ricciardo completed the top 10 over Zhou, Lance Stroll, Esteban Ocon and Sebastian Vettel.
Behind Fernando Alonso in 15th, back-to-back points scorer Mick Schumacher moved ahead of Haas’ team-mate Magnussen as Tsunoda, Kubica and Nicholas Latifi rounded out the order.