Neymar netted in the first half and crossed for Marquinhos to score after the break to put PSG on 75 points with three games left in their title chase, one point behind Lille who beat Nice 2-0 in the late game.
"It was hard, especially in the second half. Before the game we needed four wins from four games so we'll take the three points, that's the most important," said PSG centre back Presnel Kimpembe.
Lens forward Arnaud Kalimuendo was a threat to the PSG defence but the visitors lacked efficiency up front.
Just before the hour, PSG doubled the advantage as Marquinhos rose above Medina to head Neymar's whipped cross home with the hosts seemingly heading towards a routine victory.
Lens, however, made sure the reigning French champions stayed on their toes as Ganago found the back of the net with a half-volley from Jonathan Clauss's cross.
Lens, who played boldly throughout the match, left the pitch disappointed.
"There's a lot of regrets. We let them play in the first half, conceding a soft goal," said midfielder Seko Fofana.
"We tried and tried but then they scored a second. We shook them up, we played with great character."