The reigning champion and runaway leader bounced back from a 2-1 loss to Lyon at the start of the month by beating Bordeaux 1-0 last weekend, however, Thomas Tuchel's team was short of its best once more at Stade Geoffroy Guichard.
Mbappe dazzled in the 2-0 UEFA Champions League win over Manchester United on Wednesday (NZDT) but found the going tougher in a line-up without the considerable talents of Neymar, Edinson Cavani, Marco Verratti and Thomas Meunier.
Yet he settled matters with aplomb in the 73rd minute, swivelling to guide Dani Alves's lofted pass into the top corner on the volley.
It was the act of a player a class apart from the rest – much like PSG, which is 12 points ahead of Lille at the summit with two games in hand.
The same combination that undid United in midweek were to the fore again, with Angel Di Maria storming forward on the break in the 20th minute to play in Mbappe, whose shot was beaten away at the near post by Stephane Ruffier.
Mbappe felt the effects of a robust challenge from Romain Hamouma, who had a shot blocked by PSG captain Thiago Silva late in the first half after Thilo Kehrer's misjudged headed clearance.
Ruffier smothered a shot from Julian Draxler at close quarters in the 55th minute, Di Maria and Mbappe having combined on the end of Marquinhos' excellent raking pass.
Juan Bernat's replacement Colin Dagba was quickly into the action with a last-ditch challenge to deny Saint-Etienne wing-back Gabriel Silva, with Kevin Monnet-Paquet going close from the resulting 63rd-minute corner.
Di Maria should have done better when he skewed wide two minutes later, leaving Mbappe to majestically show him how it was done.
He showed a more pragmatic side as he joined substitute Layvin Kurzawa in being booked for kicking the ball away in stoppage time - PSG closing out a result that denied Saint-Etienne a move into the top four.