The deal upgrades terms from his previous contract, which was due to expire in 2027 and signed when Mainoo was still 17. Reports suggest his base wage will rise to around £135,000 per week with performance bonuses on top.
The renewal is a sharp reversal of fortune for a player who asked to leave on loan last summer after former manager Ruben Amorim restricted him to a single league start all season.
Since Michael Carrick took charge following Amorim's sacking in January, Mainoo has started 11 of 12 games, completing 90 minutes in all but one. The form earned him a recall to the England squad under Thomas Tuchel.
The contract contains no release clause, according to Sky Sport Germany's Florian Plettenberg — a detail that will reassure United supporters after the uncertainty of recent months.
Mainoo, who joined United's Academy at six and grew up in Stockport, said the club has always felt like home.
"I have grown up seeing the impact that our club has on our city, and I relish the responsibility that comes with wearing this shirt."
"We can all feel the momentum building inside the club. I am determined to step up and play my role in helping Manchester United to regularly fight for major trophies in the years ahead."
The midfielder has made 98 appearances for United, scoring the winning goal in the 2024 FA Cup final against Manchester City. He also represented England in the Euro 2024 final in Berlin.
Director of Football Jason Wilcox said the club have full belief in Mainoo's development.
"His technical ability, dedicated professionalism and humble personality make him the perfect role model for our young players and a true credit to our outstanding Academy system."
United are currently 11 points clear of sixth-placed Brighton in the race for Champions League qualification, with four Premier League games remaining.