Lomax, 23, has already made more than 100 first team appearances for Saints and is now committed until the end of 2017.
He said: “It’s a real thrill to commit my long-term future to my home town club, particularly at a time when the future of the Saints is looking so good and when Super League is clearly going from strength to strength.
“We have an improving and exciting Saints squad in an improving and strengthening Super League. I can now only look forward with ambition and excitement.”
The academy product has inherited the number one jersey from club legend Paul Wellens in the off-season, after being shifted to full-back by new coach Nathan Brown last season.
Saints chairman Eamonn McManus said: “This is a major boost for St.Helens and, on the back of other very recent extensions for our top young British players, a ringing endorsement of all the fantastic commercial and financial strides that Super League and rugby league have recently achieved and will now continue to achieve.
“There can be no better tangible evidence of these recent major achievements than the long-term retention of the very best British talent in which we as a club have invested so much and for so long.”