The deal is a £55m initial payment plus £5m in performance-related add-ons.
Sky Sports News understands the add-ons are strongly dependent on appearances and success. United have offered the England international a five-year contract with an extra year option.
Mount has one year left on his contract at Stamford Bridge, and United initially had three bids rejected by their Premier League rivals.
In a farewell social media video to Chelsea supporters on Tuesday evening, Mount said: “Hi Chelsea fans. Given the speculation over the last couple of months, this may not come as a surprise to 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 is right for me at this moment in my career.
Meanwhile, United remain in talks with Atalanta over the signing of striker Rasmus Hojlund. A gap in valuation exists – but it is thought the player wants to make the move.
A new goalkeeper also remains on the agenda. Inter Milan’s Andre Onana is one of a few United are looking at. However, there is a feeling he may be too expensive, and United continue to look at other options.
Dialogue is still open with David de Gea over a new deal, with the 32-year-old Spaniard officially out of contract.
