The then 25-year-old arrived for a medical before his £37.1million transfer from Chelsea.
A few days later, the World Cup winner was confirmed on a deal until 2018 to become United’s record signing, ahead of Dimitar Berbatov.
“The time has come for a new challenge,” Mata said. “I am excited at the chance I have to be part of the next phase in the club’s history.
“Chelsea is a top club and I have many friends there but you cannot turn down the chance to join Manchester United. I look forward to helping the manager and the team be successful in the coming years.”
Mata had won the Champions League, Europa League and FA Cup while at Stamford Bridge.
He joined a United side managed by David Moyes but the former Everton boss left Old Trafford in April after being sacked.
It was a mixed time in Manchester with Mata winning the FA Cup – scoring in their 2016 win over Crystal Palace – League Cup and Europa League but United continued to fail in the Premier League.
He made 284 appearances, scoring 51 goals, before leaving when his contract expired last year to join Galatasaray.