Only a West Brom win at Old Trafford could secure the championship for Pep Guardiola's side this weekend, but a close-range header from Jay Rodriguez in the second half proved enough to end United's slim hopes.
United's loss handed Manchester City the title and also meant the Sky Blues have equalled a top flight record.
Give it up for Manchester City, the 2017/18 Premier League champions.
Second-place United dragged the title race out for at least another week by winning 3-2 at City last weekend, only to gift wrap the trophy for City by losing to a team that is destined for relegation and had just one league win since August.
"I think we were deservedly punished because we were masters in complication", Mourinho said, throwing his players under the bus.
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.
West Brom had dropped 8 of the past nine matches and then last won on January 1-3.
Jay Rodriguez proved to be the most unlikely of title-deciders when he stooped low to head the Baggies in front on 73 minutes.
In that match, Mourinho's side produced a poor first-half display before they rallied to deny City what would have been the sweetest of coronations as champions. "Everything was slow or one more touch, one more flick, one more turn, and ball lost".
City boss, Pep Guardiola had doused high expectations over the fixture's outcome when he said he would rather be playing golf than bother about the game.
While still nine points from safety, it was an awesome end to a match that had started with Livermore showing composure that belied his role as a midfield enforcer as he cut away from Victor Lindelof to be denied by De Gea.
"You win titles with consistency at every level, not just the performance level but the mental level and the day by day". It's important we continue to fight and work hard for each other.
"I'm proud for players and staff and way they have acquitted themselves today. I am pleased we managed to show everyone that today".
"I saw lots of people on the moon because we won against City. You could see the difference in the attitude to players compared to last week", said the Portuguese coach. He name-checked eight games - Milan against Napoli, Schalke against Borussia Dortmund, Celtic against Rangers, P.S.V. Eindhoven against Ajax - but not the one that could decide the title.
In the past five seasons no side has picked up more points at Old Trafford than West Brom, who have taken 10 over that period.
Mourinho suggested before the game he was "surprised" they are bottom of the Premier League, but the time for the "too good to go down" debate has been and gone.