Sanchez joined United from Arsenal as part of player plus cash deal with Henrikh Mkhitaryan.
The Chilean said he had joined “the biggest club in the world” and couldn’t resists the chance to work under new boss Jose Mourinho.
Sanchez penned a long-term deal worth £500,000-a-week that includes a £20m singing on fee, while Manchester United have paid Arsenal another £20m to secure his signature.
He said: “I am thrilled to be joining the biggest club in the world.
“I have spent three-and-a-half wonderful years at Arsenal and I bring with me very positive memories of that great club and its fans. 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.”
Sanchez also blasted former Gunners centre back Martin Keown for branding him a “mercenary” and revealed he had sought advice from club legend Thierry Henry about the move.
“There are people (Keown) who have spoken with no knowledge of what happens inside the club and cause damage,’ he said.