United announced on Friday evening they had reached a deal to sign the Spanish playmaker, who will reportedly cost the club £37 million.
The 25-year-old’s fee will surpass that paid for Dimitar Berbatov, who cost United £30.75m when joining from Tottenham in 2008.
Mata arrived in dramatic fashion on Saturday afternoon as he stepped off a helicopter at Carrington to undergo his medical tests.
David Moyes was on hand to meet the two-time Chelsea Player of the Year, who was dressed in a smart suit with a pocket square.
Several sources reported on Friday that the Mata deal would be delayed by Chelsea's signing of Mohamed Salah, who is seemingly being brought in to provide cover, with that deal being held up in turn by the need for a medical, personal terms and a work permit for the Egyptian.
That process is expected to take several days, though there are no obvious reasons why it should not go ahead, and United will expect to complete a deal for the Spanish playmaker imminently.
United tweeted news of Mata’s arrival as they close in on the biggest transfer in the club’s history.
Since joining from Valencia for £23.5m in August 2011 Mata has twice been voted Player of the Year at Stamford Bridge but slipped below Eden Hazard, Willian and Oscar in the midfield pecking order this season.
Chelsea being prepared to sell Mata, twice their Player of the Season and a huge fans' favourite, to rivals United has raised eyebrows and even led Arsenal manager Arsene Wenger to suggest Chelsea are "playing" with the transfer window.
Wenger's point was that Chelsea have already played United twice this season so will not have to face Mata in a red shirt.
Predictably, Mourinho dismissed Wenger's claims and launched an attack against what he thinks is Arsenal's preferential treatment in the fixture list.
"Wenger complaining is normal because that's what he does all the time," Mourinho said.