The Monaco loanee, whose move is set to be made permanent for next season, netted on debut to put PSG 2-1 ahead after Benoit Assou-Ekotto had been sent off for a strong challenge on the teenager.
Edinson Cavani scored twice, while Neymar and Lucas Moura were also on target for Unai Emery's men, affording Mbappe – who fired home after his attempted pass to Neymar fell back to his feet from Ivan Balliu's failed clearance – an enjoyable maiden outing for the title favourites.
"It was a beautiful goal for my first," the 18-year-old told Canal + . "I wanted to make a pass to Ney, [but] the defender intercepted and I was quicker to react than my opponent and shot with power.
"I always said I wanted to play with great players. I'm evolving with the best in the championship and perhaps in Europe. I learn from their movements and professionalism to do my best on the field.
"All I wanted was to be on the field, the coach made that decision. I enjoyed it. I am happy, I am happy with what I did."
Metz had been causing their star-studded visitors problems before Assou-Ekotto's controversial second-half dismissal, with Emmanuel Riviere having cancelled out Cavani's opener.
Questioned on whether the former Tottenham defender deserved to see red after apparently winning the ball, Mbappe – who rolled around somewhat theatrically afterwards – added: "When you are in action it is hard to judge.
"I did not see him coming, he tackled me and it could have been more dangerous.
"Red? I do not know, but it was dangerous. I do not blame him, it was probably in the spirit of the game."