Seen as an excellent coup for the English club, Fitzgibbon has signed a three-year deal that will give him plenty of job security.
At 29 years of age, it could be the end of his NRL career, which has seen him score 31 tries in 112 games for the Knights - the club he made his way through the junior ranks at.
Fitzgibbon said he was excited to switch to the United Kingdom with his best years still in front of him.
"They play a really good brand of rugby league in the UK," Fitzgibbon said in a club statement.
"At 29 I want to come over while I still have my best years in front of me."
The loss of Fitzgibbon for the Knights comes as the club prepare to welcome star English duo Kai Pearce-Paul and Will Pryce for the 2024 season, with both signed on two-year deals. While Pryce will battle for a spot in the halves, Pearce-Paul is a second-rower and at just 22 years of age, will likely fight for the place vacated by Fitzgibbon.
Knights director of football Peter Parr labelled Fitzgibbon a "champion" of the club.
"Lachlan has been a champion of the club," Knights Director of Football Peter Parr said.
"We cannot thank him enough for what he has done at the Knights and we wish him all the best in the next part of his journey."
According to The BBC, Fitzgibbon consulted with former Warrington players Michael Monaghan and Kurt Gidley before putting pen to paper, with heac coach Daryl Powell labelling the second-rower a "great signing."
"Lachlan is a quality player and is a great signing for us for a lot of reasons," head coach Daryl Powell said.
"I've had really good, positive conversations with him. He's a top-class back-rower who's been consistent over there in the NRL for a number of years now. He runs great lines and is a great professional."