The 24-year-old should now stay at the club until at least November 2018.
McNally joined Leigh from Huddersfield Giants, and made his debut for the Lancashire outfit in 2012.
He has scored 53 tries and kicked 20 goals in 92 appearances for Leigh. He was alread contracted until November 2016, but this new deal is an improvement on that contract.
“Whenever I negotiate with players I ask them to trust my honesty that I will always do the right thing by them,” said Leigh Centurions head of rugby Derek Beaumont.
“If a player is on a contract that they have outgrown I will remedy that to ensure they are comfortable and happy.
“With regards to Gregg both Paul Rowley and I didn’t believe that the contract we had negotiated for him did him justice based on his current achievements.
“I wanted to address that and at the same time gain Gregg’s continued trust by extending his stay at the club.
“Gregg has bought into the journey we are on and has moved his wife and family from Yorkshire to live in Leigh.
“That shows a firm commitment to our club and the skills and commitment he has demonstrated in pre-season training have led Paul and me to reward him by offering him improved terms on his existing contract and to extend it by a further two years.
“Gregg is a fast developing young player who would have quickly been a target for other clubs and I am delighted that he has shown his trust in Paul and the club by agreeing to this new contract.
“It may seem a little strange in our sport to many and perhaps even unprecedented to approach a player and give him more than he had agreed to.
“I like to think this demonstrates the honesty and family values of our club to do the right thing.
“I feel the same situation exists with one of our other players and I will be looking to address that next.
“If I feel going forward that the situation happens around other players I will act accordingly.
“This enables players to feel challenged and fresh with opportunities to improve, even if they are on long-term contracts and I think that’s incremental in maintaining ambition.”
Leigh head coach Paul Rowley added: “I am delighted to add Gregg to a growing list of players secured on long-term contracts.
“Gregg was one of my original signings when I began my tenure as head coach and so I am very pleased to be able to continue to work with him as we strive to progress to the sport’s biggest stage of Super League.
“His skills and ability are there to see every week but in addition to all that talent he is a fantastic lad and a very popular team member and I am sure his team-mates will be as delighted by this news as I am.”