Running in second place behind leader Max Verstappen, Leclerc attempted to give third-placed Sergio Perez a slipstream in the final laps to help him pull clear of Russell, who was on course to give Mercedes second in its fight against Ferrari.
Perez needed to finish five seconds clear of Russell after being handed a five-second penalty for a clash with McLaren's Lando Norris.
But while Leclerc let Perez past into second, the Mexican was unable to pull clear of Russell and dropped from second to fourth after the penalty was applied post-race.
It meant Mercedes kept second in the constructors' championship by three points.
"It was all in the last few minutes because Checo had five-second penalty," Leclerc explained his assistance to the Red Bull driver.
"We had to help him in a way for him to finish in front of George with the five seconds. I tried to give him the DRS and a slipstream, but that unfortunately wasn't enough.
"It is just a shame that we finished third in the constructors' [championship]. That's all that mattered to me in this end of season, and we didn't achieve that."
From second on the grid Leclerc had two opportunities to pass polesitter Verstappen on opening lap, once at Turn 1 and once on the back straights, but the Monegasque just stopped short of pulling off the move.
"I obviously wanted to try and get that first place, but we also know that in the race, we lacked some pace compared to them," he said about the start.
"So, even if I would have passed Max there, I probably would have got overtaken again three or four laps [later].
"And at the end, my only target was to beat the Mercedes, so I didn't want to lose too much tyre juice and also time with Max. But yeah, it was fun."
Russell said he appreciated Leclerc "kept in clean" by not trying to hold him up after letting Perez past.
"It was tight with Checo at the end, I thought Charles was going to back me up but respect to him for keeping it clean," Russell added.
"[Second] means a huge amount. There are so many people back at a factory, Brackley, Brixworth who have worked so hard to achieve this.
"It's been a really challenging season. I've let the side down a couple of times this year, so pleased to bring it across the line today."
