Lorient, who had conceded 16 times in their previous four matches, triumphed thanks to goals midway through each half from Gregory Bourillon and Benjamin Corgnet.
Lille, who had won their last three league games, stay seventh with 20 points from 13 games, three pionts behind leaders PSG.
Lorient, who kept only their second clean sheet of the season, move up a place to 10th with 18 points.
PSG, who lead Marseille on goal difference, take on in-form Stade Rennes at the Parc des Princes on Saturday.
At the Moustoir, Lorient went ahead in the 24th minute when defender Bourillon headed home from a Yann Jouffre free kick.
Lille came close to equalising early in the second half but Fabien Audard produced a great save to keep out Aurelien Chedjou's header.
Lorient doubled their tally in the 69th with Corgnet beating Mickael Landreau from close range after collecting a through ball from Jeremie Aliadiere after a counter attack.