The hosts went ahead after only nine minutes when Nene poked the ball home after a Park Chu-young header bounced off Vedran Runje's right post.
Nene sealed a straight-forward win by converting a penalty in the 67th minute for his seventh goal this season after Park was brought down in the box by Romain Sartre.
Monaco moved on to 18 points after nine games, two behind leaders Olympique Lyon who slumped to a 2-0 home defeat by Sochaux on Saturday.
Lens, who have not won in the league for almost two months, dropped to third from bottom on eight points.
"The first goal was very important. Then, we held them off although they were dominating," Monaco coach Guy Lacombe told his team's website.
"We changed things in the second half and once we scored that second goal it became complicated for Lens."
Paris St Germain's title bid suffered another blow when they lost 1-0 at Toulouse and slipped to ninth, seven points off the pace.
Defender Albin Ebondo netted the winner 16 minutes from time after being set up by Moussa Sissoko as Toulouse jumped to 12th on 11 points.
PSG were without Turkey striker Mevlut Erding and midfielder Claude Makelele, who had not fully recovered from a virus.
France striker Andre-Pierre Gignac dedicated Toulouse's victory to the fan who died after being beaten by Partizan Belgrade fans at a Europa League game on Sept. 17.
"It's our first home win since Brice Taton died. This victory is for him and for his family," Gignac told French TV channel Canal Plus.
Lorient stayed sixth on 17 points courtesy of a 4-1 home thrashing of Nice, who are second from bottom.
Arnold Mvuemba opening the scoring six minutes from the interval with Marama Vahirua doubling the home side's tally three minutes into the second half.
Nice threatened a comeback when Loic Remy pulled one back shortly after the hour but Sigamary Diarra and Kevin Gameiro wrapped it up for Lorient in the last 15 minutes.
