England international Mainoo, who turned 21 this month, has agreed an improved five-year deal and completed the paperwork at the club's Carrington training centre on Wednesday.
His previous contract, which was agreed shortly before he broke into the United first team, was set to expire in the summer of 2027, though there was an option to extend it by a further year.
"Manchester United has always been my home; this special club means everything to my family," he said.
"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.
"I have the privilege of living my dream every single day, with the same relentless desire to succeed here as when I joined my first training session at the age of six.
"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."
All but back in Champions League - yet big decisions loom at Man Utd
Get news, insight and fan views on your Premier League team
