McCarthy-Scarsbrook has been a regular in Keiron Cunningham’s side all season and scored nine tries.
“I’m honoured to be at this club and delighted to extend my time at the Saints,” he said.
“I’ve enjoyed my time here and look forward to the next couple of years.
“I have been part of the club for a long time now and still have a lot of hunger and drive to succeed. We won a double in 2014 and we all want to taste that again.
“I’m also excited about passing on my experience and passion to the younger players in the squad as they continue to develop and come through the ranks.
“The fans have been superb this season and I’d like to thank them as well as the chairman, the board and Keiron for their support.”
Cunningham said: “Louie is a real leader in the group and I’m delighted he has agreed to extend his time at the Saints.
“He has been at the forefront of the pack this year and his versatility is vital to the squad.
“He also sets a good example to the younger members of the squad and that is important as we continue to develop and grow as a group.”
McCarthy-Scarsbrook joined Saints from Harlequins in 2011 after making nearly 100 appearances.
He made his international debut for England against Wales in 2008 and has gone on to play 178 times for the Saints, scoring 36 tries.