Baggies forward Jay Rodriguez's close-ranger header in the 73rd minute was the only goal on the night as the Red Devils struggled to click in attack, with star man Paul Pogba putting in yet another underwhelmin performance.
A week after City surrendered a 2-0 lead to lose 3-2 to United and squander the chance to wrap up the title in ideal style against their arch-rivals, it was relegation-bound West Brom who served it up on a plate.
Next up for West Brom is a home game against Liverpool on Saturday, and nothing less than a win will do, if they are to have any chance whatsoever of avoiding the drop down to England's second tier.
"You can't take (winning the league) for granted, I've won three now but there's so many we've missed", he said.
United have 71 also with five to play.
Manchester City's swaggering march to the title has seen Pep Guardiola's side hailed as one of the finest teams in the Premier League era. Mourinho has not delivered the Premier League title in his second season in charge, and the failure to progress to the FA Cup final is likely to increase the negativity around the Portuguese's managerial position.
Relegation to the Championship is still nearly a certainty, with nine points separating them from 17th placed Swansea City, but the mood within the West Brom camp would have at least been lifted after yesterday's win.
And it was achieved by playing a brand of football that has had the purists purring all season with City racking up 93 goals so far and on course to top the 100-point barrier. "We have spent a lot of money to have these fantastic players, to make that possible".
Only two other players have captained more Premier League title winners - Chelsea's John Terry (five titles) and Manchester United's former captain Roy Keane (four).
Ryan Giggs, who won 13 titles with Manchester United, believes City need to maintain their dominance for several seasons to join the all-time greats. "You can't describe it.
So happy to be able to share another trophy with our fantastic fans!" tweeted City's captain Vincent Kompany.