It is reported that Sanchez has signed a four-and-a-half-year deal with Jose Mourinho's side, while Mkhitaryan has agreed a three-and-a-half-year contract in north London.
Sanchez spent three-and-a-half-years at Arsenal and it appeared as though he was heading to Manchester City before Pep Guardiola's side ended their interest in the 29-year-old due to costs, but Sanchez insists he has always dreamed of playing for United and believes he has signed for the biggest club in the Premier League.
On his part Mkhitarya described his move to the Emirates as a dream come true.
Below are some of Kenyan football fans' reactions to the swap deal.
Not a single Arsenal player has come out to wish Alexis Sanchez goodbye on their social media accounts. Tunnel burst up next time we play Manchester United is on the cards. Nailed on.
— Danny Welbeck (@WelBeast)
Alexis Sanchez moved from 6th to 2nd position without playing a football game.
Is God not wonderful?
They came to our home, beat us 3-1, did a stupid dance infront of our fans then walked away with our star player. Don't ever ask me why I HATE Manchester United so much.
Sanchez akiwa Arsenal hakucheza hata Flute....but after kusign na Manchester United anacheza Grand Piano..