Thomas Tuchel was without seven first-team players due to positive coronavirus tests, with Kylian Mbappe, Neymar, Angel Di Maria, Keylor Navas and Mauro Icardi among those missing, and their makeshift XI failed to break down the hosts.
Lens went closest in the first half as Ignatius Ganago struck the right-hand post after fine individual work, but otherwise PSG controlled proceedings, seeing 78 per cent of the ball in the opening 45 minutes, even though they did not threaten particularly often.
PSG continued to dictate the ball but struggled to create chances and a Marcin Bulka error gifted Lens the lead in the 57th minute, with Ganago's goal ultimately proving decisive.
Despite the unfamiliar look of the PSG side, Lens were unable to put them under a great deal of pressure, though Ganago did go close in the 17th minute when he hit the post after leaving the visitors' defence in knots.
3 - Ignatius Ganago has scored three goals against Paris in Ligue 1, his favourite opponent in the top-flight. Hunter. pic.twitter.com/oaVzf3NGhE— OptaJean (@OptaJean) September 10, 2020
At the other end, Kays Ruiz-Atil showed plenty of promise for PSG, the 18-year-old winger shooting just wide with a nonchalant attempt from just outside the penalty area.
The other teenager in PSG's starting XI, Arnaud Kalimuendo, nearly sniffed out a chance just before half-time as he looked to latch on to Marco Verratti's lofted pass, but Jean-Louis Leca rushed out of the goal to end the danger just in time.
Bulka was significantly less commanding in the PSG net just before the hour, however, as he squandered possession from a short, simple pass, and Ganago took full advantage as he smashed past the youngster to make it 1-0.
It should have been 2-0 soon after, but Simon Banza attempted a diving-header from point-blank range when it made more sense to use his foot, and Bulka made the block.
Florian Sotoca should have done better with a header across goal 11 minutes from time, but PSG never looked like equalising as they succumbed to defeat.
What does it mean? PSG unsurprisingly struggle
It would be fair to say that many considered Lens – only promoted from Ligue 2 last term – as favourites for this encounter, given PSG's situation.
While they saw way more of the ball than Lens - 78.1 per cent to 21.9 - they had immense difficulty creating chances, getting just one shot on target across the 90 minutes.
As a result, PSG's run of scoring in 32 successive away Ligue 1 matches – a joint-record – came to an end.
Ganago keeps the PSG defence on their toes
A recent arrival from Nice, the Cameroonian was a real livewire, having three shots and touching the ball second-most often among Lens players before his late withdrawal.
He was unlucky to hit the post in the first half and then went on to get the winning goal, his pressing paying off handsomely. He has netted more (three) against PSG in Ligue 1 than any other side.
Bulka to blame
The young goalkeeper was only playing because of Navas' illness and Sergio Rico's ineligibility, but in the end he was culpable for the defeat. He gifted Lens the winner, with this other two saves ultimately worthless.
The action comes thick and fast over the coming weeks for PSG, who are next in action against Marseille in Le Classique on Sunday, their second fixture in a run of four matches in 11 days. Lens go to Lorient on the same day.