Raheem Sterling was in sparkling form, plundering a hat-trick, with Gabriel Jesus and Sergio Aguero also on target in a game that suggests the rest of the league will have their work cut out stopping Pep Guardiola's side marching to a third consecutive title.
Next weekend, West Ham travel to Brighton.
Sebastian Haller made some excellent touches and the Hammers did put City inder a little bit of pressure, especially down the flanks, but once the first goal went in, there never looked like a way back.
"We knew before the game that it would be a hard game against Manchester City", said shellshocked manager Manuel Pellegrini.
Already 2-0 ahead, Jesus thought he'd scored a third for City, only for referee Mike Dean to disallow the goal for offside following a consultation with VAR.
Mo Salah doubled the lead in the 19th with a characteristic left-foot finish from a tight angle after a nice set-up from Roberto Firmino and the game was virtually over in the 28th when Virgil van Dijk nodded home a Salah corner to make it 3-0.
"They were clinical. They've had six or seven chances and scored five goals".
Mahrez featured for the duration of the game along with Issa Diop, who ended with the losing side.
And the Spaniard admits VAR system could take joy out of the game, saying: "You have to celebrate a goal when it's clear".
By the end the Hammers were crushed and could have lost by more, but it is hard to be too critical.
Pellegrini did have some mitigating circumstances though.
As well as suffering the joint-heaviest defeat of his brief managerial career to date, Lampard was dealt the biggest reversal by a Chelsea boss in their first game in charge since Danny Blanchflower oversaw a 7-2 loss at Middlesbrough in December 1978. "It will never be comparable with what we lived last season".
Guardiola said: "Maybe the intensity and passion will leave".
Perhaps West Ham fans should take this result with a pinch of salt.
Tottenham Hotspur, the only side to keep pace with City and Liverpool last season before falling away and finishing a distant fourth, began their campaign with a 3-1 home victory over Aston Villa - Harry Kane scoring two late goals.
The last time they won a Test series in England, Steve Waugh was the captain.