For just under 27 years, the job of steering the United ship was left to Sir Alex Ferguson. But with his fellow countryman now retired, the responsibility has fallen to Moyes.
Even though today is his first day, it won't be like being at school. There will be no 'getting to know you' period followed by play time and lunch - his in-tray will already be bursting at the seam and he will have to hit the ground running.
So, with Moyes's family pictures now propped up on his desk at United's Carrington training base, Sports Mole looks at five tasks that will require his immediate attention.
The future of Wayne Rooney
Is he staying? Is he going? Does anyone actually care anymore? Moyes has to. This has been rumbling on ever since Ferguson dropped Rooney for a Champions League tie against Real Madrid in early March. Even on what was his last match in charge at Old Trafford, Ferguson still found the time to aim a parting shot at the striker by claiming that he has handed in a transfer request - his second in three years. It brought an immediate negative reaction from the majority of United fans, who sided with their retiring gaffer.
What Moyes needs to decide is if Rooney's relationship with the club is beyond repair. The player himself is reportedly open to the possibility of staying, but he would want a new contract and official clarification from the club that he did not express his desire to leave. With Ferguson still on the board, you have to ask yourself if that is likely.
It is also believed that Rooney is keen on gaining assurance that he is a valued member of the squad who will play in his preferred attacking role. Towards the end of his tenure, Ferguson lodged him in the centre of midfield. It wasn't exactly a match made in heaven, put it that way.
If true, Rooney wants all this from a man with whom he was involved in a legal dispute a few years back over quotes from his book. Moyes, who gave Rooney his opportunity as a teenager at Everton, eventually accepted an out-of-court offer from the forward.
Whatever the outcome, it's not a situation that Moyes can afford to let become toxic. No doubt he will want to nip it in the bud swiftly, mainly because if Rooney does leave, he would need to sign a replacement.
© PA Photos
Arrivals and departures
Having been used to having his hands tied at Goodison Park, Moyes will be like a kid in a sweet shop right now. United may have debt, but they still have transfer funds that their new manager could only have imagined in his dreams at Everton.
He will need to spend wisely, though, because until he has proven himself at least, every player that he recruits is going to be heavily scrutinised.
Whatever he decides to do, Moyes will be watched like a hawk by supporters, pundits and journalists alike. No pressure!
© PA Photos
Back in the summer of 2007, Ferguson overhauled his midfield options by signing Owen Hargreaves, Anderson and Nani. The trio then went on to play their part as United won a Premier League and Champions League double.
Giggs is another year older, Scholes has retired (again) and Cleverley for large parts last term failed to deliver - highlighted by the fact that he spent a fair amount of the title run-in among the substitutes. Then there is Paul Pogba - the one that got away.
This has left the United faithful desperate for fresh blood in the centre of the pitch. Thiago would be a start, but he's a creative player. The general feeling is that the Red Devils need an athletic player with bundles of energy and the ability to break up opposition attacks.
© PA Photos
Who plays at centre-back?
When his United players report for pre-season training on Wednesday morning, Moyes will find that he is spoilt for choice in the centre-back department. Rio Ferdinand, Vidic and Jonny Evans provide the experience, while Phil Jones and Chris Smalling offer not only youth and talent but pace as well.
It begs the question, why, if the reports are true, is Moyes happy to have Garay on board? Does he need him? The logical answer is that at least one of the quintet will be sent packing - Vidic to AC Milan perhaps.
Then Moyes will have to decide on a partnership. Obviously the sheer number of games over the course of a season, coupled with injuries and loss of form, means that he will chop and change from time to time.
But when the big games come around - Manchester City, Liverpool, Chelsea and the like - he will need a settled rearguard. For years it has been Vidic and Ferdinand, but injuries appear to have slowed down the ageing duo.
One would assume that if Garay was to join, because of the reported £15m fee, he would take up one of the two places.
© PA Photos
Dealing with the media
Moyes has been handling the English press ever since he was appointed the manager of Preston North End in 1998. No disrespect to them or Everton, but United are a much, much bigger animal.
Every single word that he says to the media will be watched intently by reporters and fans across the globe. People will want to hear his opinion and then criticise or praise it as they see fit.
A week on Wednesday Moyes is expected to meet with journalists for the first time as United boss. It will be no surprise if all sorts of questions are fired his way regarding Rooney, Baines and co. What is important is that his answers are methodical and well thought out. During the early part of his regime at least, Moyes needs the media to be on his side.
That is still nine days away, though - a lot can change during that time period, especially at Man United.
© PA Photos