The Portuguese coach has put pen to paper on a three-year contract with the Red Devils after days of speculation linking him with the vacant Old Trafford job.
United have been searching for a new manager since they opted to sack Louis van Gaal on Monday afternoon following the club’s fifth-placed finish in the Premier League and FA Cup triumph.
Mourinho has an option to extend his deal with United until at least 2020, as he prepares for an immediate return to the Premier League.
His appointment was confirmed via a statement on United’s website on Friday morning.
Mourinho said: “To become Manchester United manager is a special honour in the game.
“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 been out of work since December after his sacking by Chelsea following the west London club’s disastrous start to their Premier League title defence.
MORE: Man United latest news
MORE: Arsenal latest news
MORE: Chelsea latest news
MORE: Liverpool latest news