After months of speculation and days of negotiations that were held up due to image rights, the former Chelsea boss signed a three-year deal at Old Trafford to succeed Louis van Gaal, who was sacked by the club on Monday.
The Dutchman steered United to the FA Cup in his final match in charge, but ultimately his failure to secure a top-four finish in the Premier League was his undoing.
Mourinho will now look to restore the club to their former glories, though, and here Sports Mole looks at whether the 53-year-old is the right man for the job.
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A proven winner
For all that can and will be said about Mourinho, one thing cannot be disputed - he knows how to win.
It is impossible to argue with his record over the last decade and more, having won eight league titles in four different countries, in addition to lifting the Champions League twice - a tally that already matches that of Sir Alex Ferguson.
Those comparisons to Ferguson will be rife throughout Mourinho's time at the club, and it is a spectre that arguably weighed too heavy on both David Moyes and Van Gaal.
Moyes arrived at the club having done an admirable job with Everton but without a major trophy to his name in management. Following the most successful manager in British football history was always going to be hard, and it was no real surprise when the Scot was relieved of his duties after less than a full season in charge.
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In contrast, Van Gaal had the experience and the silverware that suggested that he could take United back to the top of English and European football having won 12 major honours in three different countries, but once again the legacy of Fergie, and particularly his style of play, contributed to Van Gaal's departure.
The Dutchman did provide the first silverware of the post-Ferguson era when he led the team to FA Cup glory last weekend, but it is a trophy that would have been third on their priority list at the start of the campaign and ultimately it wasn't enough to save his job.
Mourinho has an even greater pedigree than Van Gaal, and at a much younger age too. The Portuguese boss has won more trophies at the age of 53 than even Ferguson, and also leads the legendary Scot when it comes to Premier League win percentage.
Even accounting for his dreadful title defence in the season just finished, which saw Chelsea lose nine of their first 16 league games to cost Mourinho his job, he boasts a win ratio of 66% in England's top flight - the most of any manager to have taken charge of four or more games.
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Last season was very much a blip for the self-proclaimed 'Special One', and he will be desperate to prove so at Manchester United. He has won the Premier League title in three of his five full seasons in the division, and could be the man to bring number 21 to Old Trafford.
A perfect fit?
From the moment he went racing down the Old Trafford touchline in typically rambunctious style during a Champions League last-16 clash between his Porto side and Ferguson's United, there has been a begrudging respect for Mourinho amongst the red half of Manchester.
He may not have endeared himself to them in that moment, or indeed in many moments since, but his exploits throughout his career earned that respect and many United fans were pining for his appointment in the immediate aftermath of Ferguson's retirement.
On paper it certainly seems like a good match. United, for all their recent troubles, remain one of the biggest clubs in the world, and Mourinho, for all his recent troubles, remains one of the biggest managers in the world.
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He took charge of Chelsea when Roman Abramovich expected the world and more, and went on to manage two more of Europe's biggest clubs - Inter Milan and Real Madrid - where the expectations are arguably even higher.
The challenge of taking over at United will not be a daunting one for him, then, and other elements suggest that the fans will take him to heart quickly. He enjoys a better record against Liverpool than any other team in the Premier League, and also only lost once in all his meetings with United.
It is perhaps not quite a match made in heaven, though. One of the main gripes under Van Gaal was his slow and uninspiring style of football, with United scoring just 49 league goals all season - nine fewer than their next lowest tally in the Premier League era. Old Trafford as a ground also saw just 36 goals all season - the fewest in the division.
There are more stats that prove how Van Gaal's side struggled to capture the imagination of the fans too, with the Red Devils recording more backward passes than any other side in the division and also being amongst the worst offenders for sideways passes and fewest shots attempted.
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Mourinho does not exactly have a reputation as a free-flowing manager himself, with his Premier League-winning sides of the past all being built on a sturdy defence first and foremost. His Chelsea sides have only once been within 10 goals of the league's highest scorers, but in every single one of his full seasons in charge they have boasted the best defensive record.
He has been accused on numerous occasions of parking the bus and grinding out results in unattractive fashion, although such a style of play is easier to accept when it produces wins.
Of course, occasions sometimes call for a backs-to-the-wall job in order to win three points, but from the Busby babes through the 'Holy Trinity' and to the modern day, United has been built on attacking football and that must be the priority for Mourinho if he is to truly get the fans onside.
Quick fix or long-term solution?
In an ideal world, United fans would want another Ferguson - a manager who will stay at the club for a long time and guide them to unprecedented success both domestically and abroad.
The same applies for every club in the world, though, and while Mourinho may bring the success with him, realistically he is unlikely to usher in a new long-lasting era at Old Trafford.
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It is notable that he has signed a three-year contract with United as that is usually Mourinho's shelf-life at a club. He has never spent longer than three-and-a-half years in any of his managerial jobs, with his longest spell coming during his first stint at Chelsea between June 2004 and September 2007.
For the most part, his departures have not been down to a lack of success, but rather a souring of his relationships with others at the club. He was sacked by Chelsea first time after falling out with Abramovich, and at both Real Madrid and his return to Chelsea there was talk of unrest amongst the players.
There are perhaps fewer egos in the current United dressing room than at either of his previous two clubs, but at board level there are some big characters that hold great influence at Old Trafford - most notably Ferguson and Sir Bobby Charlton, two men who were reportedly opposed to Mourinho's appointment for some time.
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Mourinho has also had a love-hate relationship with the press during his time at Chelsea, at first being a dream for headline writers with his enigmatic style but then becoming prickly and uncooperative with reporters before and after matches.
Van Gaal was another who got on the wrong side of the press at times, and the pressure on him was ramped up by those persistent reports suggesting that he was on the verge of the sack before that fate eventually befell him - albeit months later than initially speculated.
The long-term future of both Mourinho and United remains uncertain, then, but the honeymoon period should at least be enjoyable for both. The Portuguese boss has a fine record in his debut seasons at a new club, steering Porto to the treble, Chelsea to a Premier League and League Cup double, Inter to the Serie A title and Real Madrid to the Copa del Rey in his respective first full campaigns.
Silverware may well be forthcoming for United next season, then, and there is still hope that Mourinho may be ready to settle down. Upon his second appointment as Chelsea boss he outlined his intention to stay at the club for a long time and, while results ultimately put paid to that idea, now might be the time for Mourinho to attempt to build a lasting legacy at a single club.
Will he give youth a chance?
For all of Van Gaal's failings, his own legacy at United is likely to end up being the number of young players who broke through under his watch.
Jesse Lingard earned a regular spot in the first team and ended the campaign by scoring a stunning winning goal in the FA Cup final, Cameron Borthwick-Jackson and Timothy Fosu-Mensah both impressed at the back and Marcus Rashford burst onto the scene in February and made such an impact that he has been included in England's provisional squad for Euro 2016.
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United is a club with a proud record of bringing youngsters through, from the Busby babes to the Class of '92, and while Van Gaal's hand was forced through injuries when it came to his integration of new faces, he then kept them in the side when the treatment room started to empty.
Mourinho, in contrast, does not have a good record when it comes to giving youth a chance. The most striking example of that comes at Chelsea, who snapped up most of the best youngsters in the country after Abramovich's arrival and boast one of the most impressive Academies in the world.
However, the last player to break through and earn a regular starting role in the Chelsea first team was John Terry back in 1998 and, while full blame cannot be attributed to Mourinho for that fact, he certainly didn't help matters with his spending sprees.
The likes of Shaun Wright-Phillips and Asier del Horno were brought in for bloated sums during his first spell in charge, while his return saw unsuccessful big-money moves for Andre Schurrle, Mohamed Salah and Marco van Ginkel.
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Ruben Loftus-Cheek was arguably the one youngster to be handed a chance towards the end of Mourinho's tenure, and even he failed to make a regular impact on the first team despite Mourinho's prediction that he would.
The Portuguese is expected to be handed another hefty war chest upon his arrival at Old Trafford, and with a number of big names already linked with the club, the likes of Rashford, Lingard and Borthwick-Jackson will be waiting nervously to see whether their breakthrough seasons have earned them a place in Mourinho's plans.
It is also worth noting that Mourinho's record of signings is far from all bad, with the likes of Didier Drogba, Michael Essien, Nemanja Matic and Diego Costa all coming to the club under his watch, but he does tend to look for proven players elsewhere than giving those already at the club a chance to shine.
Mourinho will not take charge of his first match as United boss until the International Champions Cup game against Borussia Dortmund on July 22, and won't even have his first press conference until earlier that month, but there is still plenty for him to busy himself with in the meantime.
A rebuilding job needs to be done at United, despite Van Gaal spending more than £250m on new signings during his underwhelming tenure. Personnel will be first on Mourinho's agenda, and there have been no shortage of names linked to Old Trafford even before his appointment was confirmed.
Perhaps the biggest of those surrounds Zlatan Ibrahimovic, who is out of contract this summer and eager to move to the Premier League before the end of his career. He has a good relationship with Mourinho from their time together at Inter Milan and didn't exactly quell the speculation suggesting that he is on his way to Manchester when questioned at a recent press conference.
Karim Benzema and Gonzalo Higuain have also been touted as potential targets for the new United boss, while reports have suggested that he could look to raid former club Chelsea for Matic and Willian.
At the back, there has been talk of big-money moves for Raphael Varane and John Stones, although it is likely to be one or the other from those two given their similar style and the amount it would take to prise them away from Real Madrid and Everton respectively.
The news of Mourinho's arrival would not have been warmly received by Juan Mata, who was shipped out of Chelsea by the Portuguese boss and could find himself heading for the exit door again.
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Most other players may get a chance to prove themselves to Mourinho, but the likes of Marouane Fellaini, Adnan Januzaj and Memphis Depay have all been linked with moves away from the club and would see their positions become even more tenuous should the expected influx of new players arrive.
Once the campaign gets underway, Mourinho must come to terms with a lack of Champions League football for the first time in 14 years as he prepares United for their Europa League campaign, although the last time he managed in Europe's second-tier competition he led Porto to UEFA Cup glory in 2002-03.
The first priority as far as the Premier League is concerned, as it was when Van Gaal first took over, will be to return United to the Champions League via a top-four finish. Mourinho is likely to want the title straight away, but next season is shaping up to be the most competitive yet and it could be too big a job in too short a time for him to manage that.
One of the most mouth-watering aspects of Mourinho's appointment was the prospect of the Manchester derbies next season, as he comes up against former El Clasico rival and incoming Man City boss Pep Guardiola once again.
The pair had an incendiary relationship in Spain but are widely regarded as the two best managers in world football and, with the likes of Jurgen Klopp, Arsene Wenger, Antonio Conte, Mauricio Pochettino and newly-crowned Premier League champion Claudio Ranieri in the mix, it promises to be a memorable season.
The ultimate aim for United is, of course, to regain their spot at the top of English and European football - a perch that Ferguson so famously led them to - and Mourinho's track record suggests that there is no better person currently available to do that.
Whether he is in it for the long haul remains to be seen, but in the short term he will look to restore the fear factor for United and his appointment will certainly raise optimism amongst the club's fans that the good times will be returning to Old Trafford.