A goal from Asamoah Gyan 10 minutes from time condemned Le Mans to their seventh successive away defeat as they remain second from bottom of Ligue 1.
Kader Mangane gave Rennes an early lead before Rolland Lamah levelled for the visitors but Gyan's late decider means Le Mans are still searching for their first away point this season
Le Mans were by no means outclassed, though, and created the first chance of the game when Mathieu Dossevi's close-range shot was blocked by Nicolas Douchez in the Rennes goal.
However, the hosts went ahead after just eight minutes when Le Mans goalkeeper Didier Ovono completely missed Sylvain Marveaux's corner and Mangane knocked the ball into an empty net.
Rennes were dominant and Marveaux almost doubled the lead when his header skimmed past the post while Ovono blocked Moussa Sow's header.
Gregory Cerdan had a powerful shot saved for Le Mans but they were grateful to Ovoni in added time at the end of the first half when he pulled off a stunning save from Sow's close-range shot.
The visitors made the most of their reprieve when Lamah levelled eight minutes after half-time, the Belgian striker unleashing a shot from the edge of the area from Anthony Le Tallec's pass.
Le Mans then almost went ahead but Darry Herold Goulon's shot went just past the post.
They were to rue that miss after 80 minutes when Ismael Bangoura played the ball to Gyan who converted to keep Rennes in mid-table.
