The Baggies stunned the Red Devils courtesy of an opportunistic second half goal from striker Jay Rodriguez, thus confirming Manchester City's status as Premier League champions. City had previously won an FA Cup, two league Cups, FA Community Shield, and two Premier League titles since the take-over of the influential and affluent United Arab Group in 2008.
City, however, now sit on 87, and would reach 102 points if they were to win each of their remaining five games, thus becoming the first to ever crack 100 in the English top flightin the process.
A serial victor as a manager with Barcelona and Bayern, Guardiola ended his first year in England at City without silverware - his only failure to win a trophy since his managerial career started back in 2007.
Mourinho claimed the result and subsequent celebrations affected his players' performances in a woeful display against bottom-of-the-league West Brom.
Paul Pogba, in particular, was a major disappointment following his two-goal performance in the derby.
The relationship between Pogba and Mourinho has certainly been over-analyzed and over-saturated with coverage this season.
You could see the attitudes of some players, masters of complication.
"Unbelievable season for us", said midfielder Kevin De Bruyne, perhaps City's standout player over the past eight months. There was no fluidity in our football.
'I don't think we deserved to lose but not to win this match, we deserved that punishment for sure. "Paul's game was complicated like many others, always one more touch".
'Come on, ' he added.
"I'll tell you what I told him after the match", Mourinho told multiple national newspapers.
Pep Guardiola's chargs needed one more win to secure the title following their 3-1 victory over Tottenham Hotspur on Saturday, but United's loss to West Brom means they have now mathematically clinched the title.
"I think United will win".
It's not fair, for them.
"The Champions League is seven knockout games". I'm not going to come here with excuses. I was unfortunate enough to play against that quality United side that dominated in the '90s, but this side would have beaten them - that's how good they are.
There were wins for Crystal Palace and Huddersfield Town in their bids to avoid relegation, while Burnley continued their charge to a Europa League place with victory over Leicester City.