Formula 1 icon Hamilton clocked a fastest lap of 01:20.117 seconds at the Temple of Speed, 0.169 seconds faster than Leclerc, who won last year's race for the Scuderia.
Hamilton, a seven-time world champion, has had a disappointing first season at Ferrari, not only not winning a GP but not even claiming a single podium finish.
The 40-year-old trails championship leader Oscar Piastri by 200 points and has a five-place grid penalty for this weekend after failing to slow down in a yellow flag zone on his way to the grid at the Dutch GP.
However, a strong weekend on his team's home turf in front of their adoring Tifosi could spark a resurgence for the Brit.
He was fastest in a session which was interrupted for around 10 minutes after French rookie Isack Hadjar, who claimed his first F1 podium last weekend at the Dutch GP, slid into the gravel and sprayed it onto the track.
McLaren's Piastri did not participate in the first practice, with Irish youngster Alex Dunne taking his place and finishing nearly 1.5 seconds behind Hamilton.
Piastri's teammate Lando Norris, who at 34 points back in the drivers' standings is Piastri's closest title challenger, was 0.904sec off the pace in sixth.