The 28-year-old prop has made 83 Super League appearances for Saints since joining them ahead of the 2011 season on a four-year deal which was due to run out this year.
"Louie is a very good player who is popular amongst his team-mates," Saints coach Nathan Brown said. "He also has a lot of scope for improvement and his career is on the rise."
McCarthy-Scarsbrook made two England appearances during his time in London and hopes to force his way back into the international arena.
"I've been up here for a few years now and I'm settled," he said. "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."
- Sports & Recreation