Lomax, who scooped the Super League Young Player of the Year award, appeared 29 times last term, scoring on 9 occasions.
“I’m more than happy to be signing a long-term deal with my home town club,” Jonny said. “Although there were two years left on my existing deal, I was keen to extend it.
“There’s a real excitement in the team and in the town at the moment with our new stadium about to open and we can’t wait to be playing in it. We have a strong and exciting squad for next season and are hungry for success – it’s going to be special to be a part of it.”
Chairman Eamonn McManus stated: “At the age of 21, Jonny is already one of the best players in Super League and is only going to improve further. We are proud that he is yet another product of our Academy and has committed himself unconditionally to the Saints, to Super League and to Rugby League.
“It’s fitting that Jonny is the first signing at Langtree Park as he is a future star in our new era.”
Head Coach Royce Simmons added: “Jonny is a great young talent and a pleasure to coach. He’s already proven himself to be of international calibre and will grace the England team and St Helens for many years to come.
“He’s one of a number of great young players that have come through the Saints’ Academy and have already proven themselves capable of performing at the highest level.”
Lomax made his debut for the injured Paul Wellens at Wakefield in March 2009 and has gone on to make 53 appearances for the club.