The match was in doubt after Champions League finalists PSG had seven players, including star duo of Neymar and Kylian Mbappe, test positive for coronavirus.
Lens took full advantage on their return to the top flight as Ignatius Ganago punished goalkeeper Marcin Bulka"s error to net the winning goal.
The match was played in front of 5,000 fans at Lens" Stade Bollaert-Delelis.
PSG have won seven of the past eight top-flight titles including last season when the campaign was ended early because of the coronavirus pandemic. Thomas Tuchel"s side finished 12 points clear at the top of the table.
Brazil forward Neymar, compatriot Marquinhos and Argentina internationals Angel di Maria and Mauro Icardi were all missing for the visitors.
It proved costly as third-choice goalkeeper Bulka presented the ball straight to Ganago to fire home the only goal of the game in the second half.
Reflecting on the game, Tuchel didn’t seem overly surprised by the result.
"I don"t think we could have done any better tonight," Tuchel told Canal+. "I was happy with the first half we had. We controlled the game completely, and we had possession.
"We struggled to create scoring chances today. No opportunities? Yes, I agree. But we have a small group, and we are missing the players who make the difference.
"With a young team, we made a decisive mistake and we couldn’t take advantage of their mistakes. A draw is the best we could have hoped for."
PSG host Marseille in the league on Sunday and must wait to see if any of the missing players will be available for selection.
