The 53-year-old agreed personal terms to take over the reins at Old Trafford on Tuesday but image rights negotiations meant an announcement was delayed.
Following Louis van Gaal’s sacking earlier in the week, the Red Devils have moved quickly to wrap up a deal for Mourinho.
The former Chelsea boss’ arrival had been the subject of speculation for months as a result of United’s poor form over the course of an underwhelming Premier League campaign.
The final nail in the coffin for Van Gaal was his side’s failure to achieve a top-four place and qualify for next season’s Champions League.
It will be Mourinho’s third managerial stint in the Premier League, after two spells at Stamford Bridge with varying degrees of success – the Special One has been out of work since December following his sacking by Chelsea.