Given that the visitors had conceded first in a pre-round league-high nine games this season, it was perhaps surprising to see them race into an 11-minute lead here.
A thumping 25-yard drive by Vincent Marchetti clattered against the post, only for the ball to bounce invitingly at the feet of Willem Geubbels, who duly tucked home his first league goal in nine games.
That sparked Lille - who had lost three of their four games before the international break - into life, and Giroud almost immediately had a towering header saved, while Romain Perraud’s low, bobbling effort was expertly tipped to safety.
Stephane Gilli’s men had only kept two clean sheets all season, so it seemed unlikely they would be able to keep the visitors at bay until full-time.
In fact, the lead did not even last until the break, as the evergreen Giroud equalised when he was picked out by Perraud’s pull-back and converted from near the penalty spot after being left unmarked.
The striker nearly had another moments later, but this time fired straight at Kevin Trapp from close range, while the goalkeeper made further stops from Hamza Igamane and Nabil Bentaleb before the interval.
Momentum was with the hosts after the restart, and they came within a whisker of going ahead when Trapp made another fine stop, this time sticking out a strong left hand to keep out Chancel Mbemba’s header.
At the other end, Marchetti provided a reminder of his side’s attacking capabilities as he burst onto a through ball and saw a stabbed shot blocked to safety by Berke Ozer.
That was only Paris’ second shot of the match, outlining the pressure they had been under at the other end, but their luck was about to run out, as after Giroud had forced another expert save out of Trapp, Les Dogues were awarded a penalty for a foul on Hakon Haraldsson by Marchetti.
Giroud was tasked with taking it, and he emphatically found the top corner, sending the goalkeeper the wrong way in the process.
It was no more than Lille deserved, and they appeared to have put the game to bed just three minutes later when Aissa Mandi headed into an empty net after Trapp came for a corner and got nowhere near it.
Lohann Doucet halved the visitors' deficit late on to set up a tense finale, but Lille were equal to the task and restored their two-goal lead in stoppage time from the penalty spot after Doucet was penalised for handball, with Marius Broholm on target this time.
The win lifts them into the top four and stretches their unbeaten H2H record to seven matches. Paris, meanwhile, have lost four of their last six games and sit just three points clear of the relegation zone.
Flashscore Man of the Match: Olivier Giroud (Lille)