The forward - who will turn 33 early in 2027, the last year of his new deal - looks likely to finish his career at the Auckland club.
Barnett has thrived under Warriors coach Andrew Webster since being granted a release from his Newcastle contract to move across the Tasman last season.
Able to play either through the middle or on an edge, Barnett was a member of the forward pack that pushed the Warriors to a preliminary-final berth last year.
This season, he is averaging 151 metres per game, more than in any other campaign of his career.
In the absence of the injured Tohu Harris, Barnett captained the Warriors for inspirational wins over Penrith and the Dolphins last month that put their season back on track.
He is an outside-the-box option to join Michael Maguire's 17 for the crucial second Origin match on June 26, having been selected in NSW's extended squad for the series opener.
If picked in the team on Sunday, Barnett would become only the second Warrior to play for NSW, after Ryan Hoffman in 2015.
Barnett's new deal is a boost for a Warriors side hoping to snap their hoodoo against Melbourne at home on Saturday.
The Kiwi side have not defeated the Storm in 15 attempts, last tasting victory in 2015.
Barnett credited a change of scenery and Webster's influence for his purple patch.
"I needed a bit of a change. My form and my career were stalling a bit at Newcastle," he said.
"I wanted to test myself and get out of my comfort zone. I wanted to come here to show my value.
"Webby has come in and I have really connected with him. He just gets it. He gets rugby league. He gets life. He gets the mixture."
Webster said Barnett's recent selection in the NSW squad was a fitting reward for the forward's form amid a tough start to the season.
"Mitch absolutely epitomises toughness, resilience and leadership," the coach said.
"Through the tough period we endured, Mitch was the glue. He worked so hard and really deserved his Origin call-up.
"He ticks all the boxes. The harder it gets, the better he gets."
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