The France international, who had been heavily linked with a blockbuster move to Real Madrid, ended months of speculation over his future earlier by penning a new deal with the Ligue 1 champion until 2025 on Sunday (AEST).
Mbappe marked the occasion in style with a treble against Metz, which had Boubacar Traore sent off in the second half as it was consigned to the drop at Parc des Princes.
Neymar was also on target along with Angel di Maria, who was given a great reception when he was substituted in his final appearance for Mauricio Pochettino's side before a proposed move to Juventus.
There was already a party mood for the PSG's final game of the Ligue 1 season after the news that Mbappe would remain in the French capital, with the deal confirmed before the game kicked off.
PSG's talisman opened the scoring in the 25th minute; racing on to Di Maria's delightful throughball and rounding Marc-Aurele Caillard before slotting into the empty net.
Ligue 1's top scorer doubled his tally just three minutes later. This time, Lionel Messi provided the precise pass, before Mbappe his finish under the dive of Caillard.
Neymar then got in on the act just past the half-hour mark. The Brazil international applied a neat finish as the ball broke kindly for him after Kiki Kouyate's brilliant sliding challenge initially denied Di Maria.
Mbappe completed his hat-trick within five minutes of the restart; dispossessing Kouyate and rounding Caillard before applying the finish for his 28th league goal of the campaign.
Sorry Metz was reduced to 10 men when Traore was shown a second yellow card for a rash challenge on Neymar, before Di Maria made it five in the 67th minute after Messi's initial strike hit the post.
Di Maria was in tears after the goal and was withdrawn before the final whistle, bringing his successful career with the club to an end on a high note and a guard of honour from his team-mates.