The two-time Champions League-winning boss' first appearance in the Old Trafford dugout will be as manager of England during a Soccer Aid match on June 10.
His first match in charge of United will be a July 2nd friendly against Borussia Dortmund, before a pre-season derby against Guardiola's City 23 days later.
Mourinho's sacking by Chelsea in December coincided with a massive dip in form at United - and rumours of the self-proclaimed 'Special One' replacing Van Gaal have been rife ever since.
It appears increasingly likely that Zlatan Ibrahimovic will be the former Inter and Real Madrid boss' first signing at Old Trafford.
The pair worked together for one season in Italy, with the Swede admitting he misses working with his former manager.
Atletico Madrid's Antoine Griezmann has become one of the most sought-after strikers in Europe after his stellar performances for Diego Simeone's men.
And the new Old Trafford boss is no different.
Chelsea's Willian - who Mourinho brought to Stamford Bridge in the summer of 2013 - is another player on the agenda.
The Brazilian would cost around £60 million ($122 million).
Mourinho tried in vain to bring John Stones to Stamford Bridge last summer, and is likely to make another attempt to persuade the Everton defender to play for him.
However, United will face stiff competition with City as Guardiola is rumoured to be a fan.
The new United chief is likely to watch England's clash with Australia tomorrow, which could feature the likes of Wayne Rooney and Chris Smalling.
While new sensation Marcus Rashford is likely to make his international debut.
Mourinho made his name as a translator at Barcelona for Sir Bobby Robson before being appointed as assistant to his successor, Van Gaal.
His first encounter at Old Trafford was during a Champions League last-16 tie in 2004 - famously running down the touchline to celebrate a last-gasp tie-winning goal by for his Porto side.
The Portuguese giants went on to shock Europe and win the competition.
Mourinho has won the Champions League again six years later and has claimed league titles in four countries.
Premier League success with United would make him only the fourth man after Herbert Chapman, Brian Clough and Kenny Dalglish to win England's top flight with two teams.
