The midfielder will have a medical, almost certainly on Saturday, and personal terms are also still to be agreed.
However, unless there are any hitches, Mata will make his debut in Tuesday's Barclays Premier League encounter with Cardiff at Old Trafford.
"Manchester United is pleased to announce it has reached agreement with Chelsea Football Club for the transfer of Juan Mata for a club-record fee," said United in a statement.
"The deal is subject to a medical and the agreement of personal terms.
"A further announcement will be made in due course."
It has seemed just a matter of time before Friday night's announcement was made, with it becoming obvious Mata would be joining United as early as Tuesday.
Although manager David Moyes refused to discuss the 25-year-old following Wednesday's Capital One Cup semi-final loss to Sunderland, sources involved in the deal had confirmed it was close to being agreed.
And the seal was put on the switch by Chelsea manager Jose Mourinho when he spoke to the media on Friday lunchtime.
"It's what he wants," said Mourinho.
"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."
It is thought United have agreed to pay £37million for Mata, smashing the club record £30.75m they paid for Dimitar Berbatov in 2008.
Voted Chelsea's player of the year for the past two seasons, Mata has failed to win over Mourinho and ended up being squeezed out due to the form of Oscar and Eden Hazard.
Yet Mourinho insists the former Valencia man leaves with his best wishes.
"We hope he does well," said Mourinho. "We are convinced he is going to do well.
"If Manchester United pay their record for a player it's because they know exactly what they want.
"They are convinced he's going to be important for them.
"In spite of probably many clubs, many boards and many managers think you can't sell a top player to a rival club, especially not in the same country you play, we feel and we think differently.
"The player deserves respect, the player deserves to be happy and to play where he wants to play."
Desperate to preserve his place in Spain's World Cup squad, Mata has jumped at the opportunity to help revive United's fortunes.
And while his arrival alone cannot guarantee Moyes will steer his club back into the Champions League places, it does at least underline the determination within the Old Trafford hierarchy to return them back to their previous status as top dogs, which has slipped so alarmingly this term.
It is patently obvious, however, that Mata's eventual arrival can only be the start of a lengthy rebuilding process, with former United skipper Bryan Robson acknowledging bolstering central midfield - from which Michael Carrick is now likely to be absent with an ankle injury sustained in midweek - is a priority.
"It is what everybody's been talking about for a couple of seasons now," Robson told Sky Sports News.
"Michael Carrick has been very consistent but everybody else has struggled to stamp their authority in there.
"I still feel it's an area we need to strengthen but it's only David Moyes and his staff who can make those decisions.
"But new signings, especially when they're top class, can give an injection of enthusiasm within the squad.
It's a new challenge for Mata and I'm sure if he does sign, he'll enjoy being in the team and being wanted."