Mount has signed a five-year contract at Old Trafford with the option for a further year. He is Erik Ten Hag’s first signing this summer.
Depending on add-ons the transfer fee can rise to £60m.
Mount had been at Chelsea since the age of six and came through the club’s youth ranks.
After loan spells at Vitesse Arnhem in the Dutch Eredivisie and Derby County, Mount broke into the Chelsea first team in 2019 and became a constant fixture under Frank Lampard, Thomas Tuchel and Graham Potter.
The 24-year-old made 195 appearances for Chelsea and scored 33 goals.
He ends his time as a Chelsea player with the UEFA Champions League, UEFA Super Cup and FIFA Club World Cup trophies to his name.
In a video posted on Instagram to announce his exit from Stamford Bridge, Mount said: “Hi Chelsea fans, given the speculation over the last six months this may not come as a surprise to you, but it doesn’t make it any easier to tell you that I’ve made the decision to leave Chelsea.
“I feel you deserve more than just a written statement, so I wanted to tell you directly how grateful I’ve been for all of your support over the last 18 years.
“I know some of you won’t be happy with my decision, but it’s what’s right for me at this moment in my career.
“I joined Chelsea when I was six years old and we’ve been through a lot together.
“Winning the Youth Cup, my Player of the Year awards, the Super Cup, the Club World Cup and of course that unforgettable night when we won the Champions League.
“I want to say thank you to the academy, Jim and Neil for being so influential to me from such a young age.
“The managers I’ve worked under, Frank (Lampard), Thomas (Tuchel) and Graham (Potter), the backroom staff, the unsung heroes of Cobham, all of my teammates over the years that have become my brothers.
“My family for the continuous love and support and most importantly you guys for sticking with me throughout. Wish you all the best.”
