St Etienne jump five spots with win
Pape Souare headed in Florent Balmont's cross at the far post four minutes from time. It was Lille's only attempt on target.
Toulouse had been forced to play with 10 men for most of the second half after striker Oscar Trejo was sent off following a second booking.
Lille goalkeeper Vincent Enyeama had made a number of saves and he pulled off another stunning stop to deny Adrien Regattin in the final minute. It was his ninth successive clean sheet.
Lille remains second, four points behind PSG. It is four points ahead of Monaco, which travels to Nantes later.
Toulouse is winless in its past four matches and has slipped to 14th.
Nice fell to its fourth successive defeat as it lost 1-0 at home to Saint-Etienne.
Mevlut Erding took advantage of a poor mistake by Nice goalkeeper Lucas Veronese - who was making his home debut - to score the only goal of the game in the 23rd minute. Veronese misjudged a back pass and, as the ball rolled past the goalkeeper, Erding showed lightning-quick reactions to run onto it and slot it into an empty net.
Nice struggled to get past a solid Saint-Etienne defense and never really troubled goalkeeper Stephane Ruffier.
Saint-Etienne moved up to sixth with its first win in four matches.