The deal for the much anticipated swap was signed on Monday, the Premier League clubs announced.
The 29 year-old Chilean was cited as saying:
"I am thrilled to be joining the biggest club in the world. 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."
The announcement video, posted by Manchester United on Twitter, has immediately rocked the social network, having been seen almost three million times in the first hour:
The Twitterati has generally lauded the move, many engaging in talks over the football clubs' relevant prospects:
Some, though, resorted to irony over the newly published video, taking the news with a pinch of salt:
United has agreed to pay Sanchez £400,000 a week and to spend around £15m in agents' fees, but midfielder Jamie Redknapp thinks that it's worth it considering the Chilean forward's incredible talent.
"Some players are born to play for Manchester United and Alexis Sanchez is one of them. Sanchez is a warrior who is blessed with incredible talent. He is a killer in the box who lives to score goals," Redknapp wrote in his column for the Daily Mail.
Following Alexis' medical on Sunday, Manchester United editor for the Manchester Evening News, Samuel Luckhurst, posted a picture of the Chilean forward in his new kit on Twitter. The image suggests that Alexis will keep his No.7 shirt despite leaving Arsenal, and has a chance of grabbing the spotlight in the footsteps of iconic United players like Bryan Robson, Eric Cantona and David Beckham.
Sanchez scored a staggering 80 goals in 166 appearances after the London club paid 35 million pounds ($48.83 million) for him in July 2014, and won the FA Cup in 2015 and 2017.
The end of August saw Sanchez almost join Manchester City, but that deal fell through when Arsenal were unable to sign a replacement.
"Alexis is one of the best attacking players in the world and he will complete our very young and talented group of attacking players," said Jose Mourinho, the United manager. Also, Mourinho expressed his thanks to Mkhitaryan and stressed the forward’s contribution to last season’s Europa League success.
READ MORE: Fans in Tears, Others Elated as Alexis Sanchez Bids Farewell to Arsenal
"I would like to wish Henrikh all the success and happiness that I am sure he is going to get,’ Mourinho said in a statement. ‘He is a player that we will not forget, especially for his contribution to our Europa League victory."