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The Ferrari driver was more than four-tenths quicker than second-best Lando Norris who was in turn all but matched by team-mate Oscar Piastri.
Carlos Sainz was the only other driver within a second of the front of the field.
Mercedes, meanwhile, were surprisingly absent from the top of the timesheets, with George Russell only seventh and Lewis Hamilton ninth at a venue where much was expected from the silver arrows.
After the extreme heat of a year ago, much cooler temperatures welcomed teams and drivers.
The ambient as Free Practice 1 began was only 20 degrees, conditions that are expected to remain constant for the balance of the weekend.
While a relief for drivers, it was similarly so for the teams as it made cooling their highly strung machines far simpler.
Equally, cooler conditions take the pressure off the tyres, a fact which had been expected to thrust Mercedes towards the front of the field.
The improved conditions were reflected in the times, with the initial laps in Free Practice 1 proving four seconds faster than the same session a year ago.
Back then, Max Verstappen set the pace at 1:27.428s, a time he eclipsed easily within the first 15 minutes of the session.
And those times came on the medium compound tyre, while Leclerc was able to go even faster on the hards.
That’s not to say it was easy, and both McLaren drivers had moments; Piastri at the penultimate corner while Norris was dealing with armfuls of oversteer at times.
As expected, however, the McLarens were showing well towards the top of the timesheets together with the two Ferrari.
Absent was Mercedes, at least after the first half hour.
Dominant in Vegas a week ago, Russell suggested the Lusail circuit would suit his car even more.
The Mercedes was competitive in Qatar last year, though the heat saw it struggle for longevity with its tyres – especially in the Sprint.
But with cooler temperatures this weekend it was expected that both Russell and Hamilton would feature more towards the front, only they never did.
Times improved over the course of the session, Sainz banking a 1:23.068s on the hard compound tyre with around a third of the session remaining.
It was only when the soft tyre running began inside the final 10 minutes that times truly tumbled, Leclerc setting a 1:22.242s.
That time suggested an improvement, from hard to soft tyre, of around seven-tenths per lap, and perhaps four-tenths over the mediums.
Seconds before the chequered flag, Leclerc went faster again with a 1:21.953s to end the session comfortably faster than Lando Norris and Piastri.
Sainz was the only other car within as second of the lead Ferrari, with Yuki Tsunoda fifth.
Russell ended the hour seventh, with Hamilton ninth, the pair 1.2s off the outright pace in a surprising result from Mercedes.
Such was the lack of relative performance, and strength of Ferrari, that it gave rise to question surrounding engine modes heading into Sprint qualifying later in the day.
