United's reported £37m move for Mata will be completed subject to the 25-year-old passing a medical and negotiating personal terms with the Premier League champions.
The transfer would shatter United's previous record fee of £30.75m paid to sign Bulgarian forward Dimitar Berbatov from Tottenham in 2008.
"The deal is subject to a medical and the agreement of personal terms. A further announcement will be made in due course."
Mata is expected to have his medical on Saturday and, unless there are any hitches, he should make his United debut in Tuesday's Premier League clash against Cardiff at Old Trafford.
United manager David Moyes desperately needed a high-profile signing to lift the gloom after a miserable start to his reign as Alex Ferguson's successor.
Since replacing Ferguson in July, Moyes has seen United fall 14 points behind Premier League leaders Arsenal, while they also crashed out of the FA Cup in the third round against Swansea and suffered an embarrassing League Cup semi-final loss to Sunderland in midweek.
He was repeatedly left on the bench and even when he cracked Mourinho's line-up he was replaced in nine of his 13 Premier League starts this season.
Mourinho has given Eden Hazard, Oscar and Willian the main attacking roles and the Portuguese coach conceded Chelsea's system does not suit Mata, who reportedly asked to leave after being substituted during a match at Southampton earlier this month.
It had seemed the deal might be delayed while Chelsea tied up their move for Basel's Egypt winger Mohamed Salah, but Mata has been given the green light to quit Stamford Bridge.
"It's what he wants," Mourinho earlier on Friday before United confirmed the deal.
"It's what he asked us to accept. Chelsea Football Club is proud to act this way, in an open way, in a human way.
"The offer, compared with what the club paid two years ago (when Mata signed from Valencia), was fantastic."