The Australian and Samoan international played 124 NRL games for the side between 2014 and 2020, playing every position on the park except for five-eighth, hooker and the front-row during his stint seven-season stint.
The back-rower is back in Australia as part of St Helens' 26-man squad set to face Penrith in the World Club Challenge next weekend, and will face-off against St George Illawarra in tonight's warm-up clash.
The 26-year-old has won more premierships in the Super League than he played finals games for the Knights, collecting three consecutive wooden spoons between 2015 and 2017, however after a series of concussions later in his NRL career, he credits his departure for his rugby league revival.
"Looking back on it now, it was probably one of those things that saved my career a little bit. It brought the love of footy back," the now-forward told The Newcastle Herald.
"This is my last year in England and hopefully there is some interest back home here and to getting back and playing with someone here, but if not - it is what it is.
"I've loved the decision of going over there. I've learnt so much and it got me out of my comfort zone. I probably got a bit too comfortable in Newcastle, if I'm honest.
"I came to a club that ... when I rocked up, there were eight English international players, so I sort of had to reinvent myself and make a name for myself, which is probably what I needed."
The club switch came as a massive culture shock for Mata'utia, who said that achieving success at a club was 'weird' after his stint in Newcastle.
"I had to be on the jump from the start, they just demand standards," he said.
"It was something that took me by shock, because I never really had that accountability. It made me a better player and taught me a lot about professionalism.
"The success was weird. Winning was normal for us, and I'd never actually had that feeling. The flip side of that is, every team wants to beat St Helens so every week is a really hard game.
"It gets a bit rough at times whereas when I was at Newcastle, we were sort of at the bottom of the table for many years and a team that was always rebuilding."
While he's off-contract at the end of this season, Mata'utia had closed the door to an NRL return, but the former Kangaroo believes you can 'never say never'.
"Never say never, we'll see how we go. I haven't locked in anything as of yet."