The new agreement will keep the 21-year-old at Old Trafford until 2031, extending his previous deal which had been due to expire in 2027.
Mainoo's new contract completed a major turnaround in his season. The England international struggled for regular opportunities under former manager Ruben Amorim and even requested permission to leave on loan last summer after falling down the pecking order.
However, Carrick's arrival in January changed Mainoo's situation completely. The academy graduate became a key figure in midfield and started 12 of Carrick's first 13 matches in charge as United climbed to third place in the Premier League.
His strong performances also earned him a recall to the England national football team squad in March ahead of this year's World Cup in the United States, Canada and Mexico.
Speaking after signing the new deal, Mainoo said he remained fully committed to helping United compete for major honours again.
"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."
Mainoo also reflected on his journey through the club's academy system after joining United at the age of six.
"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," he added.
"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."
United now prepare for a huge Premier League clash against Liverpool at Old Trafford on Sunday, with Mainoo expected to play a leading role once again in Carrick's improving side.
Bukayo Saka has said Arsenal would head into next week's Champions League semi-final second leg with confidence after securing a hard-fought draw away to Atlético Madrid in Spain.
Milos Kerkez says he is "growing" into life at Liverpool F.C. as he continues to adapt to the demands and responsibility of playing for one of England's biggest clubs.
Tottenham winger Xavi Simons has already begun his road to recovery after suffering a serious knee injury that will keep him out for several months.
Diego Simeone has insisted Atletico Madrid were the better side despite being held to a 1-1 draw by Arsenal in the first leg of their UEFA Champions League semi-final on Wednesday night.
