City had moved within touching distance of the title with a 3-1 win at Tottenham on Saturday.
Despite dominating possession at Old Trafford, Mourinho's side fell to a 1-0 loss thanks to Jay Rodriguez's goal - meaning the title celebrations could begin for United's neighbours.
At last, Manchester City's players can enjoy a serene Premier League title triumph, winning the title without even playing. In the end, none of them needed to do anything for Manchester City to be crowned Premier League champion.
Only Manchester United (1907-08 and 2000-01) and Everton (1984-85) have ever wrapped up the title with as many games remaining, while Guardiola has also become the first Spanish coach to win the English top-flight title.
Manchester City are now 16 points ahead of 2nd placed side Manchester United - the Premier League record points gap between 1st and 2nd place at the end of a season is now 18 points by Manchester United in 1999-00.
This result will not have any significant effect on the league table beyond placing the title in City's hands.
Manchester City (28 wins) are now just three wins away from this with five games to play.
"Our Time. Our City". Vincent Kompany did, with images showing the City captain celebrating at his in-laws' house where he was having dinner. They were unbeaten in the Premier League until January 14, when Liverpool ended that incredible run at Anfield.
"Champions! What a great feeling!" The best team by a street, they entered 2018 14 points clear and never remotely looked like being caught.
The 2016-17 Blues vintage only failed to take three points on nine occasions, but it appears something of a formality that Guardiola's men will break the record as their remaining games are against Swansea City, West Ham, Huddersfield Town, Brighton & Hove Albion and Southampton.
As for City, it has a month to finish a season with the most points, goals and wins in a single Premier League campaign.
Manchester United manager Jose Mourinho: "I won eight titles and I am not on the moon because I won against Manchester City". It's good for the Premier League as well. I have seen many teams win it as champions for the whole season, and it's the first case for us.
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