The France captain started on the bench but came on in the 51st minute and promptly broke the deadlock, drawing a foul from Rasmus Nicolaisen and tucking away the resulting spot-kick just past the hour mark.
Zakaria Aboukhlal equalised with a penalty three minutes from time after he was tugged back by Morocco team-mate Achraf Hakimi, leaving Luis Enrique still searching for his first win as PSG coach.
Mbappe's appearance was his first for PSG since a friendly on July 21 after not being invited to the pre-season tour in Japan and South Korea and then missing the opening 0-0 draw against Lorient.
He had been made to train with players the club are trying to offload after he refused to sign an extension to his PSG deal, but the 24-year-old was brought back into the fold last Sunday after holding "constructive and positive discussions" with the club.
The U-turn came just two days before Neymar was sold to Saudi side Al-Hilal for a fee believed to be close to 100 million euros ($109 million).
French media reported Mbappe made Neymar's departure one of the necessary conditions for him to sign a new contract in 2022, when the former was strongly linked with a move to Real Madrid.
Mbappe was joined in the squad in Toulouse by new signing Ousmane Dembele, the two close friends named on the bench for the visit to the French Cup holders.
PSG created few clear-cut chances a week ago against Lorient and were again limited in the first half, Hakimi denied by the legs of Guillaume Restes and Goncalo Ramos thwarted by a sharp reflex stop.
Mbappe and Dembele summoned
Mikkel Desler curled an attempt narrowly over for Toulouse before teenager Warren Zaire-Emery's fine strike was ruled out for offside just after the break.
Luis Enrique soon turned to Mbappe and Dembele, the latter wearing the number 10 shirt vacated by Neymar, and their impact on the game was almost immediate.
Mbappe's foot was caught by the trailing leg of Nicolaisen, the PSG forward awarded a penalty after a VAR review and sweeping home the opening goal.
Dembele's clipped ball through for Vitinha saw the Portuguese midfielder lift over the advancing Restes only for his effort to spin just wide of the post.
PSG's inability to add a second goal proved costly when Hakimi was punished for needlessly impeding Aboukhlal as chased the ball on the edge of the box.
Aboukhlal, who scored in last week's 2-1 win at Nantes, beat Gianluigi Donnarumma from the spot to snatch a valuable point for the hosts.
Lyon slumped to a second straight defeat to start the season as captain Alexandre Lacazette was sent off in a 4-1 home loss to Montpellier earlier on Saturday.
Beaten by Strasbourg last weekend, Lyon fell behind to an Arnaud Nordin goal before Jordan international Mousa Al-Tamari struck in either half for Montpellier.
Lacazette pulled a goal back but then saw red for kicking out in frustration at Teji Savanier.
Nigerian Akor Adams, who scored twice on his Montpellier debut in a 2-2 draw with Le Havre, compounded Lyon's misery with a fourth goal a minute from time.
