Jay Rodriguez proved to be the most unlikely of title-deciders when he stooped low to head the Baggies in front on 73 minutes.
Not even the most enthusiastic City or West Brom fan could have predicted this – Darren Moore's side were without a win in their last 15 away league games and United were on a run of five straight league victories.
And West Brom keep their faint hopes of survival alive, moving to within nine points of safety.
Baggies goalkeeper Ben Foster was just as much the matchwinner as goalscorer Rodriguez. Old Trafford was already celebrating with Lukaku's seemingly unstoppable header travelling towards the bottom-corner, but the ex-United man somehow kept it out.
Moment of the match
Nobody quite knew where to look when Rodriguez's header hit the back of the net. I'm not sure the man himself could quite believe it. You could almost hear City fans going wild down the road.
What the managers are saying
He said: "They won the title because they were the best team, because they won a great amount of points. They lost only two matches because they were the best team. That's the reason why they were champions."
Meanwhile, Baggies caretaker manager Darren Moore hailed his players for doing their jobs well.
Moore said: It was a good performance today and we got our just rewards in the end. We have come up against a very, very good side in Manchester United but everyone stuck to their job and did everything we asked of them."
What the fans are saying
Man Utd: de Gea, Valencia, Lindelof, Smalling, Young (Rashford), Ander Herrera (Lingard), Matic, Pogba (Martial), Mata, Lukaku, Sanchez.
Subs not used: Jones, Rojo, McTominay, Joel Pereira.
West Brom: Foster, Nyom, Dawson, Hegazi, Gibbs, Phillips (Krychowiak), Livermore, Brunt (Yacob), McClean, Rondon (Sturridge), Rodriguez.
Subs not used: Myhill, Gabr, Burke, Field.
West Brom v Liverpool, Premier League, 21/04, 19:45