Adrien Hunou gave Rennes the lead after 13 minutes in a match pitting the Ligue 1 champions - PSG - and the winners of the Coupe de France.
However Rennes could not press home their advantage and Mbappé levelled early in the second-half at the Shenzhen Universiade Sports Centre in China.
The 20-year-old tapped into an empty net after Pablo Sarabia's pass across the face of the goal.
Di Maria's winner was a sumptuous strike direct from a free kick.
The Argentine, who had replaced summer signing Ander Herrera, curled the ball over the Rennes defensive wall and into the top left hand corner of Thomas Koubek's goal.
Rennes beat PSG in April to claim the Coupe de France. Before the Champions Trophy - a clash between the French league champions and the Cup winners - Mbappé spoke about the need to avenge the Cup final defeat and unfurl a statement of intent for the impending league season.
"I mentioned that we needed to start well and we've done that," said Mbappé, who was deemed man-of-the match.
"It was a tough battle towards the end but we all dug fought for each other and it's good to come away with a trophy for the fans here in China as well as for the ones in France."
Rennes kick off their Ligue 1 campaign at Montpellier on 10 August while PSG begin the defence of their crown on 11 August against Nimes at the Parc des Princes.