Victors via shoot-out when the teams met in the Coupe de France final in April, Rennes looked set again to upstage their more glamorous opponents through Adrian Hunou's tidy 13th-minute finish.
Star forward Mbappe had other ideas and his 57th-minute equaliser laid the platform for Angel Di Maria to complete the turnaround in splendid style, the substitute stroking in a stunning free-kick 17 minutes from full-time.
PSG have now claimed the season's first silverware for seven straight years and did so on this occasion without Barcelona-linked Neymar, who watched on as he missed the match through suspension.
Thilo Kehrer headed an early chance against the crossbar and seemed to be thinking about the opportunity as he dozed off a minute later.
Hunou darted in behind the defender to steer Benjamin Bourigeaud's right-wing cross past Alphonse Areola.
PSG's start would have been worse had Juan Bernat seen red and not yellow for a two-footed lunge on James Lea Siliki.
Mbappe unleashed several fruitless first-half shots before Edinson Cavani poked wide with a poacher's effort.
The former eventually restored parity with the easiest of tap-ins, Pablo Sarabia cushioning Marquinhos' lofted pass into Mbappe's path.
Sarabia almost bundled in a second moments later and it was the Spaniard who won the foul for the 73rd-minute winner.
Di Maria took ownership of the dead ball and gave Tomas Koubek no chance with a sublime strike into the top-right corner to complete the quickfire turnaround.
What does it mean? Mbappe the figurehead for PSG front line
Quizzed about Neymar's situation ahead of the match, Mbappe - perhaps predictably - told reporters he wants his team-mate to continue on at Parc des Princes.
While that remains no certainty, World Cup winner Mbappe seems destined to become the main man in attack with or without the Brazilian around.
He was the driving force in this match, keeping Rennes defenders occupied in the first half and finding a way through them after the interval.
Sarabia impresses on competitive debut
Thomas Tuchel looks to have made a canny acquisition with his capture of winger Sarabia, who assisted the leveller, played a part in the winner and looked to be fitting comfortably into new surroundings.
Bernat lucky to escape greater sanction
Ill-discipline has been a problem for PSG in the past - they were shown four red cards in Ligue 1 last season - and left-back Bernat should know better than to go flying into challenges in such a low-key setting.
He was extremely fortunate not to be dismissed for the first-half tackle that could have forced Lea Siliki from the pitch.
Both sides now return to France to begin the new Ligue 1 season.