The Hammers had already beaten Arsenal and Liverpool away this season with 2-0 and 3-0 scorelines respectively, but few gave them a chance against title favourites City.
Manuel Pellegrini's side had won all five games in the Premier League without even conceding a goal, so West Ham shocked the world when Victor Moses smashed his side in front in the first half.
One became two shortly after, as Diafra Sakho got West Ham's second to leave manager Slaven Bilic in dreamland at the Etihad Stadium - only for Kevin de Bruyne to pull a goal back before half time and give City hope.
However, West Ham dug in and got their victory, meaning Bilic has now beaten three of the biggest clubs in England, all away from home; a quite remarkable record so far.
After the game, co-chairman Gold - who took the decision to replace Sam Allardyce with Bilic over the summer - took to Twitter to comment on the win.
Gold congratulated Bilic for masterminding another stunning victory, before praising the fans and players who made the win possible.
Well done Slaven, great result today. The fans were magnificent and the players gave everything. pic.twitter.com/MPPek9WH3X— David Gold (@davidgold) September 20, 2015
With three victories in a row and just one goal conceded during those games, West Ham have bounced back in stunning fashion after the 4-3 defeat to Bournemouth in August.
Now, Hammers fans will be dreaming of a top eight finish under Bilic, who is earning a reputation as something of a giantkiller early into his reign having beaten Arsenal, Liverpool and now Manchester City.