Olivier Giroud is just the fourth player to score for both Chelsea and Arsenal in the Champions League (also Anelka, Fabregas and Gallas). But the France veteran made the most of a rare outing with an opportunistic victor in the 91st minute.
However, either side of Hudson-Odoi's goal, Werner somehow found a way to scoop a shot from six yards over the bar and Mount saw a poked effort clawed wide by home keeper Alfred Gomis. But as the ball looped awkwardly up, the ever-alert Giroud reacted quickest to leap above static centre half Damien Da Silva and planted a bullet header under the crossbar.
Sehrou Guirassy looked as though he had secured a point for his side with an equaliser minutes earlier, but Giroud profited from some poor Rennes defending to claim the win after Callum Hudson-Odoi had put the visitors in front in the first half.
Mount picked out his fellow Chelsea academy graduate with a long pass.
England global Mount was unlucky not to double the away side's lead eight minutes later when Gomis pulled off a superb save to keep out his close-range effort following a Ben Chilwell pull-back.
Mendy was speaking after this week's win at Rennes.
Chelsea were denied a second when Werner - on target twice from the penalty spot in the reverse fixture - converted Cesar Azpilicueta's cross but was flagged for offside.
Mendy made the save of the night in the 73 minute when he got his body in the way of a powerful Gerzino Nyamsi header, and then pounced to stop a goal on the rebound.
Rennes have won only once in the past 10 games, having lost 3-0 in west London three weeks ago.