United had to at least draw to keep the title race mathematically alive but they were stunned at Old Trafford when Jay Rodriguez headed home after the break.
City manager Pep Guardiola might have missed one of the proudest moments of his career, however, after admitting on Saturday that he had a round of golf booked with his son and was not planning to watch United's game.
He would have been as surprised as everyone else that the title, seemingly a foregone conclusion for months, was rubber-stamped in such strange circumstances.
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 perfect style against their arch-rivals, it was relegation-bound West Brom who served it up on a plate.
West Brom had lost eight of their previous nine games and last won on January 13.
"We've won the league for you, Man City, Man City," West Brom's fans chanted after the final whistle.
City, who rebounded from the United loss and a European exit at the hands of Liverpool by beating Tottenham Hotspur 3-1 at Wembley on Saturday, have 87 points with five games left.
United have 71 also with five to play.
It is Abu Dhabi-owned City's third title in seven seasons and their first since former boss Manuel Pellegrini achieved the feat in 2013-14.
Another feather in Guardiola's cap
Guardiola, who failed to land a trophy in his first season in charge at The Etihad, opened his account by winning the League Cup in February against Arsenal.
But a first Premier League title, to go with the three La Liga titles he won as manager of Barcelona and the three Bundesliga titles he delivered for Bayern Munich, will be particularly satisfying.
He is the first Spanish manager to win the Premier League title — a prize he says is the hardest to claim in Europe.
Not only that but he did it with five games remaining, equalling the English top-flight record of United (1907-08 and 2000-01) and Everton (1984-85).
Guardiola and Mourinho have a long rivalry but the United manager said it was not fair to suggest the West Brom loss was the only reason for City's league title.
"They won the title because they were the best team, because they won a great amount of points. They lost only two matches [this season] because they were the best team," Mourinho said.
"That's the reason why they were champions and probably because I won eight championships I was not in the moon [sic] with the victory against them [last week] because that doesn't make me feel super happy.
"And in my opinion a fact, for sure … some of the people that were too happy just with that."