The Portuguese coach had been hotly tipped as the leading candidate to take over from Louis van Gaal since the Dutchman’s sacking by the club on Monday.
Mourinho has signed a three-year contract with the Red Devils, with an option to stay at the club until at least 2020.
And the news of Mourinho’s appointment was finally confirmed via a statement on the club’s website on Friday morning.
Mourinho joins United after he was sacked by Chelsea back in December following their disastrous start to their title defence.
United opted to sack Van Gaal on Monday afternoon after the Red Devils ended up in fifth place in the Premier League table in his second season and won the FA Cup, beating Crystal Palace 2-1 in Saturday’s final at Wembley.
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