Mbappe was set up by Lionel Messi for his 12th league goal of the campaign.
The victory moved PSG 16 points clear of second-placed Marseille, who have a game in hand.
PSG coach Mauricio Pochettino picked a full-strength team against Rennes, the only side to beat them in the league this season, but the hosts were as lacklustre as they has been several times in this campaign.
PSG's ultras displayed banners critical of the club directors, in particular sporting director Leonardo.
"Leonardo, Time to Get Lost?" one banner read.
Rather than keeping his stars fresh for the Champions League clash against Real Madrid on Tuesday, Pochettino started Mbappe and Messi against Rennes, who defeated PSG 2-0 at home.
Rennes sat deep to absorb PSG pressure and almost scored in the 14th minute when Baptiste Santamaria header flew just wide from fellow midfielder Benjamin Bourigeaud's corner.
Mbappe hit the post in the 40th, and one minute into the second half Santamaria fired just over from Bourigeaud's pass.
A Mbappe goal midway through the second half was ruled out for offside.
The visitors looked set to hold out for a point until Lionel Messi found Mbappe inside the penalty area and the French star curled a low shot into the corner.
Rennes goalkeeper Dogan Alemdar got a hand to the ball but will be disappointed that he was unable to keep it out.
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