Russell looked like benefitting from a slow stop for Alpine's Esteban Ocon and a late call to intermediates for Ferrari's Carlos Sainz to climb to third as rain hit the principality in the final third of the race.
But after his pitstop for intermediates on lap 55, Russell appeared to get distracted by a yellow flag for a lapped Lance Stroll and joined the Aston Martin driver up the Mirabeau escape road.
That not only dropped Russell behind Ocon and team-mate Lewis Hamilton, but as he re-joined the racing line he was hit by Red Bull's Sergio Perez, which cost the Briton a five-second penalty for an unsafe rejoin.
"I'm really kicking myself because P3 was almost guaranteed," Russell said when asked by Autosport about his race.
"There was a yellow flag, I backed off and as soon as I touched the brakes I locked up and followed Stroll up the escape road.
"That's probably a lesson that actually when you're not on it and you're not focused, you make those mistake.
"Probably, if there wasn't a yellow flag there, I would have just been focusing more. I wouldn't have gone off and cost the team a comfortable P3."
After his clash with Perez Russell reported strange behaviour on the rear of his Mercedes W14.
But once the car settled down and any damage appeared minimal, he suggested to the team to let him past Hamilton and protect his buffer to Leclerc, knowing his five-second penalty would be added after the finish.
"I definitely damage the car a bit. I wasn't sure I was going to be able to continue but it kind of sorted itself out as the laps progress.
"I think the toes got bent at the rear end. I felt really uncomfortable in the car, but we were the quickest on track during that period. So, I don't really know what was going on.
"I was being held up by Ocon and Lewis, and Charles was closing me down. I was definitely not going to risk anything on Lewis in conditions like this. But with that five-second penalty it could have been a good buffer if Charles had caught it up."
But with Leclerc not gaining any ground, Russell followed Hamilton home in fifth, a safe 10 seconds clear of the Ferrari driver.
"It was not needed in the end, and I said to the team, no obligation and no pressure from my side and then as soon as I knew we were safe to Charles, I just brought it home," he added.
"But yeah, it's a bitterly disappointing when you do everything right for 98% of it, but that one tiny mistake costs everything."
