Since Alex Ferguson retired after winning the Premier League in 2013 the club have finished seventh, fourth and fifth, securing their only silverware in that time by winning this season's FA Cup.
That trophy was not enough to save Louis van Gaal who was dismissed this week following two years in charge.
“I think I prefer to forget the past three years,” Mourinho told the club's TV channel. “I prefer to focus on the giant club I have in my hands now and I think what the fans are expecting me to say is that I want to win.”
The former Porto, Chelsea, Inter Milan and Real Madrid manager, sacked by the London club in December after his second spell there, believes he has the right experience to be successful at United.
“I think it comes in the right moment of my career because Man United is one of these clubs where you need really to be prepared for it,” said the charismatic Portuguese.
“Giant clubs must be for the best managers and I think I am ready for it so I could say I am happy, I am proud, I am honoured. I feel great.
“I want to win,” added Mourinho. “I need the supporters and players to feel that. (The fans) know what they can give me and what I can give them.
“I've played so many times at Old Trafford and the feeling was empathy. I remember when I won at Old Trafford with Real Madrid and I said the best team lost, not many people at Real Madrid were very happy with that.”
Mourinho did not offer any hints about whether Ryan Giggs, who worked as Van Gaal's assistant, would be staying.
There was also no mention of possible transfer targets like Sweden and Paris St Germain striker Zlatan Ibrahimovic.
Mourinho has been given a three-year contract, which British media estimated being worth between 30 million and 45 million pounds.
“Jose is quite simply the best manager in the game today,” United executive vice-chairman Ed Woodward said in a statement.
“He has won trophies and inspired players in countries across Europe and, of course, he knows the Premier League very well having won three titles here.
“His track record of success is ideal to take the club forward.”
Mourinho was widely expected to succeed Van Gaal and his agent Jorge Mendes met United officials on Tuesday to discuss personal terms.
The talks became protracted, however, when it emerged Chelsea still owned his name as a trademark.
Now the stage is set for the marriage of two of world soccer's biggest names, Mourinho and United, England's most successful club and a global giant.
“To become Manchester United manager is a special honour in the game,” Mourinho said in an earlier statement.
“It is a club known and admired throughout the world. There is a mystique and a romance about it which no other club can match.”
In two spells at Chelsea he won the Premier League three times, including their first for 50 years, and the FA Cup and League Cup.
Mourinho also won league championships in his first seasons at Porto and Inter before helping Real to their first La Liga crown in four years.
United's fifth-place finish under Van Gaal this season meant they failed to qualify for the Champions League for the second time since Ferguson left. They will play in the Europa League next term.
There are reservations about Mourinho's preferred style of play with former United striker Eric Cantona saying the club should have opted instead for Pep Guardiola who is taking over at Manchester City.
But other former players, including goalkeeper Peter Schmeichel and former captain Steve Bruce, believe he will revive the fortunes of the Old Trafford team.
Original source: Forget last three years, Mourinho tells United