The 53-year-old took a late-night train from London to Manchester Piccadilly last night, posting an Instagram video when he arrived at his destination in the early hours of Monday morning.
“I am here/UNITED we can,” he posted with the short clip.
He then headed to the Lowry Hotel with his entourage, before leaving the next morning at 7.50am to get started on his new job.
Mourinho was appointed as Louis van Gaal’s successor in May, days after the Dutchman won the FA Cup, as he paid the price for missing out on qualification to the Champions League.
The Portuguese began last season as Chelsea boss, but was sacked in December as the Blues’ title defence faltered.
But Mourinho has wasted no time in beginning to strengthen the Red Devils squad, having already signed defender Eric Bailly and striker Zlatan Ibrahimovic. Armenian midfielder Henrikh Mkhitaryan is arrive this week, while Paul Pogba is also being strongly linked with a return to Old Trafford.