The Ferrari driver was the fastest of the 23 drivers to take part in the day, logging a 1:23.510s to head former team-mate Carlos Sainz, who was in action for Williams.
Sainz had been fastest for much of the day before conditions eased in the final hour of the marathon day.
George Russell was third for Mercedes while Jack Doohan was the leading Australian in eighth.
Oscar Piastri was only 19th, after completing only the afternoon session, while Liam Lawson was 13th quickest
The New Zealander completed the full day, logging the most laps of any driver to accumulate a total of 883km – the equivalent of almost three grand prix distances.
Next best was Sainz, who logged 810km, while wearing a subtly branded white helmet. He was unable to wear his usual colours, or even team-issued overalls, as he technically remains contracted to Ferrari.
There were no such limitations for Esteban Ocon, who has been released by Alpine and was in full Haas regalia as he made his debut with the American-registered operation.
He ended the day 10th fastest with a best of 1:24.305s, almost eight-tenths away from Leclerc’s lap.
It mattered little with a wide variety of programs such that the standings were a curiosity rather than any sort of form indicator.
Like in pre-season testing, a far better indicator is the number of lap each driver and team recorded.
On that front, Ferrari took the honours with 1527km, narrowly ahead of McLaren with 1511km.
In truth, there was little between the 10 teams, with Sauber recording the fewest laps but still managing 1350km for the day between Gabriel Bortoleto and Nico Hulkenberg.
While Russell completed a full day, Frederik Vesti and Kimi Antonelli shared duties in the second Mercedes.
Antonelli had been scheduled to complete the full day, however the illness that prevented him racing in F2 over the weekend saw a reduced program for the Italian.
He managed 344km, the second lowest running of any driver, with only Vesti on 305km completing less.
The day ran without interruption barring a mysterious red flag inside the final eight minutes of the nine-hour day, and a very brief yellow flag when Lando Norris spun at Turn 14.
Precisely what the red flag was for remains unclear; no car had stopped on track nor was there any apparent debris.
It didn’t matter for McLaren, Piastri and Pato O’Ward climbed from their cars early to pose for a team photo even before the session was completed.
Lando Norris was not in the photo, having completed the morning’s running for Pirelli and had long since left the circuit.
Doohan and Alpine had a trouble-free day, the Australian logging the fourth most laps behind Lawson, Felipe Drugovich (Aston Martin) and Sainz.
His 761km of running combined with rookie Paul Aron’s 672km to leave Alpine as the fourth most prolific team of the day.
With the nine hours of running in Yas Marina complete, teams will return to their bases and begin preparations for 2025 – and 2026 given rules open up on developments of that car from January 2.
It promises to be a busy off-season, ahead of next year’s championship launch at an event in London on February 18.
Testing then kicks off in Bahrain on February 26-28 prior to Melbourne hosting the opening round of the season on March 14-16.
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