But the Baggies had other ideas as they frustrated the Red Devils and snatched the victor through Jay Rodriguez with just over 15 minutes to go.
Winning the title by two points from Chelsea, in the Champions League United saw off Barcelona in the semi-finals and, in the final, beat Chelsea on penalties in the Moscow rain.
City extended their lead to 16 points after beating Tottenham Hotspur 3-1 on Saturday and United's defeat means that, with five games still to play, the gap is unassailable.
City subsequently join the United team of 2000/01 in becoming the only Premier League club to win the title with five matches remaining.
Mourinho's team have a league game at Bournemouth on Wednesday before their FA Cup semifinal with Tottenham at Wembley next Saturday.
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For the millions (and millions) of pounds United spend on their attacking talent, with the likes of Romelu Lukaku, Alexis Sanchez and Paul Pogba starting against West Brom, the Red Devils lacked spark without their academy graduates Marcus Rashford and Jesse Lingard (in addition to Anthony Martial).
The West Brom fans didn't miss a chance to make light of the situation on a day of disappointment for the home side.
In situations such as these - with United being second in the table - Ferguson may have trusted his players to play with more freedom and blow West Brom away, which begs the question of whether Mourinho has the same faith in his squad to deliver.
Mourinho, though, was quick to dampen talk that United had handed the title to City, insisting it is their dominance over the season that secured their coronation.
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They won the title because they were the best team, because they won a great amount of points.
United manager Jose Mourinho had long given up hopes of winning the title, instead focusing on finishing second.
There were a few United half-chances before half-time, while a Dawson header back across goal from a corner was begging to be turned in by West Brom.
But after Sunday's post-match tweet, United fans will be celebrating if and when the Baggies' relegation is confirmed. The first half was a bad advert for the English top-flight. They lost only two matches because they were the best team.
Jay Rodriguez has scored three goals in his past four Premier League appearances - as many as he did in his previous 26 combined.
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