"The chance to play in this historic stadium and to work with Jose Mourinho was something I could not turn down.
"I am very proud to be the first Chilean player ever to play for United's first team and I hope I can show our fans all around the world why the club wanted to bring me here."
Mourinho welcomed his new signing, who will wear the iconic No 7 shirt at United.
He said: "Alexis is one of the best attacking players in the world and he will complete our very young and talented group of attacking players.
"He will bring his ambition, drive and personality, qualities that make a Manchester United player and a player that makes the team stronger and the supporters proud of their club dimension and prestige.
"I would like to wish Henrikh all the success and happiness that I am sure he is going to get. He is a player that we will not forget, especially for his contribution to our Europa League victory."
Sanchez also made a veiled criticism of a series of ex-Arsenal players in a statement on Instagram: "There are people (former club players) who have spoken with no knowledge of what happens inside the club and cause damage.
"I must say I always gave 100 per cent, until the last day, when I asked (the manager) to be in the team, because I wanted to (make) a contribution. I remember today, a conversation I had with Henry, a historic Arsenal player, who changed club for the same reason and today is my turn."
HOW SANCHEZ NETS £600,000-A-WEEK
Basic weekly wage: £350,000 (NZ$668,680).
This is the highest basic wage of any player in the Premier League. Manchester United have four of the top six earners in terms of basic weekly wage with Paul Pogba on £290,000, Romelu Lukaku on £250,000 and Zlatan Ibrahimovic on £220,000. Manchester City's Sergio Aguero and Yaya Toure also earn £220,000.
Image rights: £100,000 a week (NZ$191,000)
Bonuses: £144,000 a week (NZ$275,000)
