The Ligue 1 leaders, who conceded an early goal by Park Chu-young and went down to 10 men when Gervinho was sent off eight minutes before the interval, have 58 points from 30 games.
Lille made a terrible start when France defender Adil Rami fluffed a back pass and Park intercepted before dribbling past keeper Mickael Landreau and tucking the ball into an empty net after 12 minutes.
The visitors increased the pressure but had Ivory Coast forward Gervinho sent off for shoving Adriano after the Brazilian had fouled him. It was Lille's first red card of the season.
The win sent Monaco from 18th to 15th, one point above the relegation zone.
RAMI ERROR
"My mistake led to the goal," Rami told Foot Plus TV. "We did not play with our hearts.
"Some players do not deserve to play for Lille," added Rami who will leave at the end of the season to join Valencia.
Paris Saint-Germain boosted their European chances with a 2-1 win at Caen that lifted them within two points of third-placed Stade Rennes who lost 2-0 at neighbours Stade Brest.
Rennes fell behind in the 28th minute when Nolan Roux struck with a 35-metre drive and Bruno Grougi added a second goal for Brest 10 minutes into the second half from close range.
PSG took the lead against Caen when Christophe Jallet volleyed home in the 13th minute from a Nene cross. Fellow midfielder Clement Chantome then netted midway through the second half with a powerful shot from just outside the box.
Caen, who reduced the arrears in injury time through Romain Hamouma, are 17th on 35 points, one point above AJ Auxerre who slipped into the drop zone after a 2-2 home draw against Saint Etienne.
Only one point separates 14th-placed Valenciennes and Auxerre with eight matches left.
