Nimes got off to a poor start as they were reduced to 10 men after Loïck Landre was sent off for a dangerous challenge on new signing Rafinha.
Mbappe got the scoring underway on 32 minutes with a simple finish from close range, with Rafinha picking up the assist.
The numerical advantage was clear to see but Nimes had goalkeeper Baptiste Reynet to thank for keeping the score low with numerous saves.
PSG had to wait until the 77th minute for the second goal as Pablo Sarabia's knockdown found Alessandro Florenzi, who made no mistake from close range.
Mbappe was on target yet again as the French forward latched onto a pass from Sarabia before calmly slotting past Reynet for his brace.
Sarabia rounded off the scoring for the defending league champions by drilling the ball past Reynet after getting picked out by Colin Dagba.