Slaven Bilic pounced to take advantage of the striker's reluctance to sign a new deal at Old Trafford with Jose Mourinho having signed Zlatan Ibrahimovic, Anthony Martial already an established star and Marcus Rashford's emergence leaving his prospects of first team football bleak.
Fletcher has been signed to a long-term deal in East London and has Premier League and Europa League football to look forward to in the club's new 60,000 seater Olympic Stadium.
Manchester United will be entitled to compensation from the Hammers for the talented 20-year-old and Bilic will be hoping he can prove one of the bargains of the season.
West Ham's Andy Carroll
New signing keen to learn from trio of stars
The towering forward has been talking to West Ham's official website about the move and believes he can gatecrash the big time this season with a little help from new teammates Andy Carroll, Dimitri Payet and Manuel Lanzini.
“Andy is an England international so I can only learn from someone like him and take different aspects of his game and add them into my own game," a level-headed Fletcher told whufc.com.
West Ham's Manuel Lanzini celebrates with Dimitri Payet
'They can only improve me'
“The quality they (Payet and Lanzini) have is unbelievable and for me to be playing alongside players like that can only improve me and make me better as well.”
“I want to put on the West Ham shirt as many times as possible and play in the Premier League. I also want to play in the Europa League as well and help West Ham challenge for a top four finish which has got to be the aim as well."
Ashley Fletcher scores while on loan at Barnsley
Fans already like Fletcher after Millwall goal
Hammers fans have already taken a shine to Fletcher.
That's because he enjoyed a superb spell on loan at League One Barnsley last season, scoring nine goals in 27 appearances including one after two minutes of the League One play-off final victory over West Ham's hated rivals Millwall.
As any football fan worth their salt knows, there is no love lost between the Hammers and Millwall in what is one of the oldest and fiercest rivalries on the planet.