Leclerc finished just 0.071 seconds clear of Russell, with Max Verstappen third and Lewis Hamilton eighth
Formula One is in Miami for the first time, at a 3.36-mile circuit constructed around the Hard Rock Stadium, 15 miles north of the city.
And while Leclerc, who leads Verstappen by 27 points, was the fastest out of the blocks, Mercedes will take comfort from their first showing of the weekend.
The Silver Arrows have brought a number of upgrades here – including a new low-drag rear wing and revised front wing – in a bid to reverse their early-season problems, and the early signs are positive, with Russell posting a competitive time.
pourtant, Hamilton, who finished only 13th a fortnight ago, a été 0.858 sec back – although his speediest lap was compromised by traffic.
Carlos Sainz crashed out on the opening lap of the past two rounds in Melbourne and Imola, and the Spaniard suffered a front-right puncture after losing control of his Ferrari through Turns 4 et 5 and limping back to the pits.
Leclerc also suffered a low-speed spin, while Valtteri Bottas’ crash midway through the one-hour session resulted in the only red flag.
The former Mercedes driver spun at Turn 7, sliding backwards into the wall and damaging the rear of his Alfa Romeo.
Verstappen, who dominated in Imola to get his championship defence back on track, finished 0.179 sec back from Leclerc, with Serio Perez fourth in the other Red Bull.
Pierre Gasly was fifth, one place ahead of Sainz, while Alex Albon impressed to take seventh for Williams.
The concluding action of the day gets underway at 17:30 heure locale (22:30 Royaume-Uni).