The lose for United means that City team can not be displace from the top of the table, thereby given Pep Guardiola his first Premier League title in his second years as that Manager of Manchester United team.
They could yet beat three significant Premier League landmarks set by Chelsea - the highest points tally of 95 in 2004-2005, the most wins in a season at 30 in 2016-2017 and the most goals scored at 103 in 2009-2010.
The win also made West Brom just the second side to start the day bottom of the table to win a Premier League game at Old Trafford - Blackburn Rovers won 3-2 in December 2011.
"I think we were deservedly punished because we were masters in complication", Mourinho said, throwing his players under the bus. They lost only two matches because they were the best team. "One more touch to control the ball, one more flick, one more trick".
Jay Rodriguez struck the games only goal in the second half, and the Baggies held out for their first away victory since August.
Caretaker boss Darren Moore is a popular figure at Albion and has quickly introduced the kind of togetherness and organisation so stark by its absence under Alan Pardew, who was sacked at the beginning of the month.
West Brom finished with their backs to the wall but had chances of their own, and Jake Livermore had earlier tested David De Gea with arguably the best opportunity before the break.
"This one differs in that we were consistent the whole season and we were able to play at the level we're expected to perform at". Just 24 hours later, with Guardiola claiming he would be on the golf course, City were teeing off for a title party.
Since Sheikh Mansour bin Zayed Al Nahyan took ownership of the club in 2008, City has won seven trophies, adding an FA Cup, two League Cups and an FA Community Shield to their league titles.
Give it up for Manchester City, the 2017/18 Premier League champions. I was unfortunate enough to play against that quality United side that dominated in the '90s, but this side would have beaten them - that's how good they are.
Manchester United manager Jose Mourinho: "I won eight titles and I am not on the moon because I won against Manchester City".
Thierry Henry: "I am not surprised with what Pep has done in this league and previous year, I was one of the people saying stop having a go at the man because that's how he plays".