The Portuguese left home at 2.50pm on Thursday and concluded talks at a central London hotel, agreeing to succeed Louis van Gaal as the new boss at Old Trafford.
Sky Sports News HQ broke the news at 5.20pm after Mourinho returned home half an hour earlier with documents in his hand and a bottle of red wine, having met with United's executive vice chairman Ed Woodward to complete the deal.
Mourinho symbolically wore a black and red tie for the final stage of negotiations, and the deal is completed on the 12th anniversary of his winning the Champions League for the first time with Porto, who knocked out United on their run to the final.
Mourinho, who has been out of work since being sacked by Chelsea in December, replaces his former mentor Van Gaal, who was dismissed by United two days after they won the FA Cup against Crystal Palace at Wembley.
Sky sources reported after the final whistle that Mourinho was set to replace the Dutchman as soon as possible, and Van Gaal was informed of the club's decision in a personal visit by Woodward to his home on Sunday night.
By Tuesday, talks between Mourinho's agent Jorge Mendes were well under way via phone and email and negotiations were expected to be a formality and completed within 48 hours.
But Mendes and Woodward were forced to meet at United's offices in central London to rectify a number of image rights issues, including Chelsea's ownership of Mourinho's name.
Those issues were smoothly resolved over the course of Wednesday and Thursday and Mourinho emerged from his Chelsea home shortly before 3pm to secure the job he has wanted since Sir Alex Ferguson stood down in 2013.
