City had moved within touching distance of the title with a 3-1 win at Tottenham on Saturday.
They clinched the silverware 24 hours later thanks to Albion forward Jay Rodriguez’s 73rd minute header at Old Trafford.
United had denied City the chance to wrap up the title last weekend when they came from two goals down to beat their bitter rivals 3-2 at the Etihad Stadium.
But Jose Mourinho’s men still ended up handing the trophy to City with a lacklustre display against bottom of the table Albion, who had won only three times in the league this season before this weekend.
City are 16 points clear of second placed United, who can only take a maximum of 15 points from their remaining five games.
Winning the title with five matches to go makes City the joint earliest champions in the Premier League era — matching the achievement of the 2000-01 Manchester United team.
City boss Pep Guardiola had planned a day of golf with his son Marius on Sunday, so the Spaniard might have learned of his first Premier League while on the course.
Speaking after the win over Tottenham, Guardiola admitted winning the title in England would be one of the greatest achievements of his glittering career.
He won three league titles with Barcelona and twice lifted the Champions League trophy during his Camp Nou reign.
The 47-year-old also took the German title three times with Bayern Munich, but he believes the unique strength in depth and physical demands of the Premier League makes this triumph among his very best.
He also conceded it was sweet to triumph in a country where some critics claimed his purist principles couldn’t flourish.
Guardiola and his players will have the chance to celebrate properly next Sunday when they host Swansea in their next fixture.