On a night when nothing seemed to go to plan for his side, PSG head coach Thomas Tuchel saw Thomas Meunier put through his own net to give Lille the lead before Bernat pulled the visitors level.
The wing-back was then shown a straight red card for a foul on Nicolas Pepe, Meunier and captain Thiago Silva having already withdrawn due to injuries.
Pepe struck six minutes after half-time and further goals from Jonathan Bamba, Gabriel and Jose Fonte rubbed salt in the wounds of PSG, who failed to adjust to their man disadvantage.
PSG remain 17 points clear at the top of Ligue 1 and will be expected to wrap up the title against Nantes in midweek, but Lille enjoyed one night of swaggering dominance as they strengthened their grip on second place.
Tuchel's men had a Kylian Mbappe goal disallowed after two minutes but then fell behind when Meunier unwittingly diverted Jonathan Ikone's looping cross into his own net.
Mpabbe ensured the visitors' response was swift and incisive, marauding down the right flank and showing exceptional control before chipping the ball to Bernat, who stroked in a first-time shot from close range.
Mbappe thought he had scored himself after an electric burst of pace but he was flagged offside, shortly before both injuries and Bernat's rashness drastically altered PSG's prospects.
Pepe went through on goal again six minutes into the second half, when he latched onto Ikone's defence-splitting pass and rolled a finish under Alphonse Areola's body.
The lively Bamba scored the second of two superb opportunities that came his way, lashing the ball just inside Areola's near post after 65 minutes.
Gabriel, on for injured Lille skipper Adama Soumaoro, rose to head home Lille's fourth 19 minutes from time and fellow defender Fonte was similarly unmarked as he converted Pepe's corner to complete the rout.
What does it mean? Champions must wait a Lille bit longer
Tuchel will be confident of seeing his men wrap up the title soon enough, but the manner in which they fell apart following the loss of Bernat should worry him, especially given the question marks that linger over PSG's mentality following their embarrassing Champions League exit against Manchester United.
Pepe shines in front of his suitors
Pepe is thought to be a transfer target for PSG, with Tuchel singing his praises in the build-up, and he showed exactly why with an all-action performance and a well-taken goal.
With his side reduced to 10 men and deprived of key defenders, Marco Verratti failed to marshal a panicky midfield, instead disappearing as Lille were afforded the run of the game.
Key Opta facts
- Lille have picked up 64 points after 32 games in Ligue 1, their joint-highest tally at this stage of a season along with 1949-50, where they finished as runners-up.
- PSG have conceded five goals in a Ligue 1 game for the first time since a 5-1 defeat to Sedan on December 2, 2000. It is the first time since April 27, 2010 at Grenoble that PSG have lost by a four-goal margin.
- Juan Bernat scored his first Ligue 1 goal in his 21st game with PSG. He got his first red card in the "big five" European leagues in his 148th game.
- Mbappe has been involved in 34 goals in the top-flight this season (27 goals, seven assists), only Lionel Messi (45 with 33 goals and 12 assists) has contributed more in Europe's major leagues.
- Nicolas Pepe's 30 goal (19 goals, 11 assists) is the highest tally for a Lille player in a top-flight season since Eden Hazard (36 with 20 goals and 16 assists) in 2011-12.
Lille face struggling Toulouse away from home next weekend, while Tuchel's men are in action at Nantes on Wednesday – hoping it will be third time lucky when it comes to wrapping up the title, having also slipped up by drawing at home to Strasbourg last weekend.