Ivory Coast striker Wilfried Singo headed in the opener for Monaco in the 23rd minute with Aleksandr Golovin stretching the lead 13 minutes later.
Guillermo Maripan added the third just before the hour mark as Monaco eased to a third victory in four matches this season.
Monaco have 10 points and are two points in front of Marseille who were held to a 1-1 draw by Nantes on Friday.
“After five minutes, we were the bosses on the pitch,” said Monaco coach Adi Hutter.
“I’m happy because we played against our biggest opponent this season, a Champions League opponent. We deserved this victory.”
For Lens, it was their third defeat of the campaign and they are second from bottom.
It is a worrying start to the season for northerners Lens who were second last season, only missing out on the Ligue 1 title by a point to Paris Saint Germain.
They will also start their Champions League campaign in just under three weeks’ time against Sevilla.
Franck Haise’s team created very few chances on Saturday in a game where record 30-million-euro signing Elye Wahi struggled to make an impact.
“We’re a squad in dire straits,” said Haise.
Lens have now conceded 10 times in four games; last season, it took 15 matches for them to give away a 10th goal.
“With 10 goals conceded in four matches, I can’t say that I’m not worried,” added the Lens coach.
“There are currently players who are not at the level we knew, or not at the level we should expect. I want a combative team and to be much more involved from the start.”
In Saturday’s early clash, the Breton derby between Brest and Rennes ended in a 0-0 draw.