Brest, who claimed a draw at Parc des Princes earlier in the season in Ligue 1, came close to breaking the deadlock when, Martin Satriano hit the post with a fine curled effort after 30 minutes. But from there, PSG took control, Mbappe opened the scoring with a driven shot from a tight angle.
Danilo Pereira doubled their lead shortly after, and though Brest pulled one back in the second half through Steve Mounie following a well-worked counter-attack, their chances of a turnaround were ended by a silly red card for centre-back Lilian Brassier.
Goncalo Ramos then added a third in second-half stoppage-time to put the result beyond doubt and secure PSG's passage to the quarter-finals.