The victory helped the champions to stretch their lead at the top to 17 points.
Christopher Nkunku’s opener and Mbappe’s second half double maintained PSG’s 100 per cent home record in the league this season.
The goals lifted them further clear of second-placed Lille who drew with Strasbourg on Friday.
Nkunku put the capital club in front five minutes before the break when he expertly controlled Marco Verratti’s ball over the top before producing a composed finish.
Mbappe had two goals ruled out in the first half, one for offside and one for handball, but made amends after the interval.
He finished from close range on 69 minutes to mark his half century in the French top flight, before finishing off a sweeping counter-attack.
At the age of 20 years, two months and three days, the France striker became the youngest player since Yannick Stopyra in 1982 to reach the 50-goal landmark.
PSG lead the standings on 68 points with 22 wins from 25 games.
Nimes are in 11th place with 36 points.