The 28-year-old has decided to extend his stay at Langtree Park having made 83 Super League appearances for the outfit since joining in 2011.
"I've been up here for a few years now and I'm settled," McCarthy-Scarsbrook told the club's official website.
"Saints are a great club who want to win trophies and I want to achieve things in the game. I'm hoping I can realise that here. Signing an extension was a no brainer for me really.
"I came north to win things and I'm sure we can do that. I know I am not the finished article and I have to continue learning. I put pressure on myself to strive to be the best I can and I'm looking forward to this season and the future."
The former London Broncos player would have come to the end of his previous deal this year.