The 25-year-old Spain international, who had fallen down the Stamford Bridge pecking order under Jose Mourinho, has completed a club-record £37.1 million move to Old Trafford
The Spain international missed first-team training with the west London outfit on Wednesday as he closed in on the club record move worth 37.1 million pounds.
The midfielder flew to United's Carrington training base on Thursday morning to complete a medical, and is now expected to make his debut for the Red Devils against Ole Gunnar Solskjaer's Cardiff City on Tuesday.
Jose Mourinho has already moved to secure a replacement for the Spaniard, as the Blues gazumped Liverpool to agree a fee with Basel for Mohamed Salah, a transfer which could cost up to 15 million pounds.
Mata, twice named Chelsea's Player of the Season, struggled for regular football under Mourinho, with the Portuguese recently admitting his "door was open" should the playmaker seek an exit.
And United has jumped at the opportunity to bring the 25-year-old to Old Trafford, with David Moyes keen to boost his midfield options and strengthen the club's bid to claim Champions League qualification after the disappointment of crashing out of the FA Cup and Capital One Cup.
For his part, Mata - who completed 90 minutes in the Premier League just twice under Mourinho - is keen for game time ahead of the World Cup in Brazil and will receive a significant increase on the 70,000 pound-a-week salary he received at Stamford Bridge.
He enjoyed a hugely successful two-and-a-half seasons at Chelsea, lifting the FA Cup, Europa League and Champions League - assisting Didier Drogba's late leveler in the final against Bayern Munich.
Since leaving Valencia for the Premier League in 2011, only compatriot David Silva has recorded more assists than Mata's 27, while he also notched 18 league goals for the Blues.