Published: 2018-04-15 23:38:40 BdST
United had to at least draw to keep the title race mathematically alive but were stunned at Old Trafford when Jay Rodriguez headed home after the break.
City manager Pep Guardiola might have missed the moment that his first English title was sealed, however, after admitting on Saturday he had a round of golf booked with his son.
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 Jan. 13.
City, who rebounded from the United loss and a European exit at the hands of Liverpool by beating Tottenham Hotspur 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.
Guardiola has won the League Cup already for City this season 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 for the Spaniard.
"You cannot imagine," Guardiola said on Saturday. "It would be one of the best three seasons I have ever had as a player and as a manager. The amount of goals, chances and points, to do that in the Premier League, with the contenders we have, is because the players are so good."