Last year's Ligue 2 champions had to settle for a home draw against LOSC and now trail leaders Paris Saint-Germain by 10 points
Mounir Obbadi gifted Claudio Ranieri's men an early lead, but Divock Origi restored parity before the interval with a good finish.
Monaco started the match brilliantly and needed only four minutes to open the scoring. Valere Germain set up Obbadi inside the area and the 30-year-old calmly slotted home from 12 yards out to make it 1-0 for the home side.
Jeremy Toulalan then came close to making it two with a shot from just outside the area, but the experienced midfielder aimed just wide.
Lille first threatened at the half-hour mark when Idrissa Guaye tried his luck after some sloppy defending from Ricardo Carvalho, but his acrobatic attempt failed to trouble Danijel Subasic.
There was little the Monaco goalkeeper could do about Origi's attempt in the 38th minute, though. The Belgian midfielder received the ball from Ryan Mendes before finding the net with a fine finish.
Lille started the second half lively and Salomon Kalou in particular caused the hosts all kinds of trouble. The former Chelsea man found Jonathan Delaplace in a dangerous position, yet his lobbed effort was kept out by Subasic.
Monaco then took over the initiative again and young midfielder Geoffrey Kondogbia nearly restored his side's lead with a superb long-range strike after a corner, but his shot went just over the crossbar.
With 10 more minutes to play, Kalou cut inside from the right after a quick counterattack and hit a powerful shot from a narrow angle, but the forward failed to get his attempt on target.
Monaco currently sit second in Ligue 1 with 63 points from 30 games, trailing leaders Paris Saint-Germain by 10 points, while Lille remain third in the table on 54 points.