Two of the Baggies' wins came in their opening two games of the season and they have won just one since, a January home match against Brighton & Hove Albion.
Mourinho's men will be desperate to finish the campaign behind City but will now need to pick themselves up after this surprise setback. Winning titles in every league is important, with Barcelona was special.
It had been a traumatic week for City, who threw away a two-goal interval lead to lose 3-2 at home to United in the derby last weekend, missing the chance to become champions in front of their own fans at the Etihad Stadium.
Only in the last 10 days, as they lost to United and were knocked out of the Champions League quarter-finals by Liverpool, had City's quality been called into question. In sealing the league at this stage, City matched the record for the earliest title win, held by United since 2001. Guardiola's side became the first team to win 14 successive English top-flight games in a single season, prompting United manager Jose Mourinho to concede the race was probably over before Christmas.
"I saw lots of people in the moon because we won against City". I think with my experience you don't win titles with inconsistency.
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 flight in the process. As of such, Herrera is likely to get a start again, and Fellaini might hope to play a late role, having seen a full recovery.
Although, observers know that it was a matter of time before City is crown the champion of the lead.
Sergio Aguero, who became City's record goalscorer this season, added: "Ecstatic about clinching another title with this great team, and with all the support of our fans".
Few - including Guardiola - expected it to happen on Sunday. They've been by far the best team in the league.
Given that they have won only three of their 33 league matches this season, it is clear that they are going down.
Should United beat West Brom today, Bournemouth will also have the opportunity to give the blues the title when they face United on Wednesday.