Aleksandr Golovin missed an excellent chance to level before the break while Neymar was denied a hat-trick by a sharp reflex stop from Benjamin Lecomte early in the second half.
Monaco made it 3-3 with 20 minutes to go with the help of the video assistant referee system.
The linesman raised his flag as Slimani forced in from close range but replays showed Marquinhos had knocked the ball towards the Algerian, prompting the original offside decision to be overturned.
Mbappe could have won it at the death but sliced wide on the volley as PSG drew for the first time this season in the league.
Lille’s miserable away form continued as they lost 1-0 at Dijon, who played for over an hour with 10 men after the dismissal of Hamza Mendyl.
Julio Tavares scored the winning goal on 47 minutes as Dijon pulled four points clear of the relegation places while Lille blew the chance to return to the top three.
Lille are unbeaten at home but have won just once in 10 outings on the road while losing on seven occasions.