Monaco loanee Mbappe was handed a start in a fearsome-looking PSG line-up which also featured world-record signing Neymar, Julian Draxler and Edinson Cavani.
It was Cavani who opened the scoring in the 31st minute, calmly rounding goalkeeper Eiji Kawashima to slot home from Neymar's throughball.
But the visitors were stunned by Emmanuel Riviere's leveller before the interval following excellent work from Mathieu Dossevi, as the bottom club threatened to spring a surprise on the league leaders.
Riviere should have put Metz ahead early in the second half after Alphonse Areola failed to deal with a back-pass, but PSG escaped and Assou-Ekotto's 56th-minute red card for a strong but seemingly fair challenge on Mbappe turned the tide in their favour.
Mbappe was the man to make them pay, the 18-year-old marking his maiden PSG outing with a well-taken finish three minutes later, while a goal-line clearance denied him a second.
Neymar added the third with a fine 20-yard finish and Cavani netted again – substitute Lucas Moura bundling in the fifth – as PSG christened the 'MCN' trio that may well rival Barcelona's legendary and now defunct 'MSN' triumvirate.
PSG predictably started on the front foot and Yuri Berchiche's cross into the box enabled Draxler to force a save from Kawashima, before Cavani was flagged for offside as his follow-up effort was also repelled.
The home side had a let-off when the Thomas Meunier's pass somehow found its way to Cavani, who again looked to be beyond the last defender, but this time Kawashima's right-hand post denied him.
It was not all one-way traffic and Renaud Cohade's teasing delivery had to be cleared by Marquinhos, with Riviere poised to pounce.
The former Newcastle United striker then turned smartly in the area and fired just wide as Metz began to grow into the game.
But the hosts' burgeoning confidence suffered a significant setback when Neymar flexed his creative muscles to slide Cavani through on goal, the Uruguay international providing a cool finish when Mbappe was also on hand if needed.
Mbappe and Cavani linked up when the Frenchman's cross with the outside of his boot was headed goalwards by the latter, only for Kawashima to produce an acrobatic save.
PSG were made to pay for that profligacy at the other end, Dossevi beating a couple of players before seeing his chipped delivery headed in from close range by Riviere.
Kawashima was kept busy as Unai Emery's men sought to restore their advantage before the interval, with Neymar's low strike tipped wide by the Japanese.
Opposite number Areola got away with inexplicably heading Marquinhos' back-pass across his own goal when Riviere failed to find the unguarded net from 10 yards out.
And then came the pivotal moment, with Assou-Ekotto sent off despite clearly winning the ball in a crunching tackle on Mbappe, who wasted little time in rubbing salt in the wounds.
Ivan Balliu's attempt to clear Mbappe's ball into the box fell to the teenager, who lashed home to put his new side ahead just before the hour.
He almost doubled his tally a minute later, but Moussa Niakhate made an excellent block on the line, before Cavani struck the underside of the crossbar with a powerful strike.
Not to be outdone, Neymar got in on the act 21 minutes from time, steering a measured finish past Kawashima, and Cavani's tap-in for his seventh league goal of the season put the result beyond doubt soon after.
Lucas added further gloss with an untidy finish, his initial attempt coming back off the post before he managed to stab it in to remind Emery that he too can be an asset in PSG's star-studded squad.
Key Opta facts:
- PSG have picked up five wins after five Ligue 1 matchdays for the first time ever.
- Edinson Cavani has scored 15 goals in his last 13 Ligue 1 games. The Uruguay international is the first player to score in each of the first five Ligue 1 matchdays since Sylvain Wiltord with Bordeaux in 1998-99.
- Neymar has scored and delivered an assist in the same match in three of his four Ligue 1 games, as many as in his last 47 LaLiga outings.
- Kylian Mbappe found the net with his first shot on target as a PSG player in Ligue 1.