The former Chelsea and Real Madrid boss has been handed a three-year deal at Old Trafford, ending months of speculation linking him to the United job.
The 53-year-old replaces Louis van Gaal, who was sacked on Monday despite leading the club to their first FA Cup win in 12 years at the weekend.
Announcing the appointment, Ed Woodward said: “José is quite simply the best manager in the game today. He has won trophies and inspired players in countries across Europe and, of course, he knows the Premier League very well, having won three titles here.
I’d like to take this opportunity to welcome him to Manchester United. His track record of success is ideal to take the Club forward. Mourinho has been an open admirer of Manchester United for many years and has been expected to replace Van Gaal since his sacking by Chelsea in December 2015.
“To become Manchester United manager is a special honour in the game,” the Portuguese said. “It is a club known and admired throughout the world. There is a mystique and a romance about it which no other club can match.
“I have always felt an affinity with Old Trafford; it has hosted some important memories for me in my career and I have always enjoyed a rapport with the United fans. I’m looking forward to being their manager and enjoying their magnificent support in the coming years.
Mourinho has eight league titles to his name from his days with Porto, Chelsea, Inter and Real Madrid, including three Premier League triumphs.
But United have not finished higher than fourth in the three years since Sir Alex Ferguson retired after collecting 13 league crowns in his 26-year reign.
They finished in fifth place in 2015-16, prompting the decision to dismiss Van Gaal and replace him with his former Barcelona coaching protege.The club have yet to confirm whether assistant manager Ryan Giggs will remain as part of Mourinho’s backroom staff.