PSG capitalized on a fast break in the 26th when Neymar found Achraf Hakimi, who fired into the top corner.
Kylian Mbappe's decision to stay with PSG was widely celebrated in the French league, even by coaches who still have to defend against the prolific forward next season.
Kylian Mbappe won the French league's best player award for the third time in his career on Sunday, then diplomatically swerved a question about his future.
Marseille trailed runaway leader Paris Saint-Germain by 12 points but kept a three-point cushion over third-place Rennes.
Lille caused PSG problems but defended poorly.
PSG has needed injury-time goals despite an armada of attacking talent and a huge squad, and produced another late winner at home.
Nantes were third after 10 games and stayed in the top five well into the second half but a plunge in form has dropped them to 10th.
Apart from a freak loss to promoted Strasbourg this month, the super-rich star-studded team has been nearly flawless, amassing 50 points in 19 games.
Clubs in the top two divisions in France spent a record 697 million euros in the close season, the French league announced.
According to LFP, this year's total amount of purchases were four times the amount of 2016 - reaching a total of $830mns of investment.