Roy Hodgson celebrated his first win as West Brom boss as Birmingham suffered a massive Carling Cup hangover in a 3-1 defeat at St Andrew's.
Goals from Youssuf Mulumbu, James Morrison and Paul Scharner enabled the Baggies to leapfrog over their local rivals in the battle for Barclays Premier League survival.
All City could manage was a consolation effort from Jean Beausejour and they paid the price for some untypically sloppy defensive play.
Scharner produced a superb performance in the centre of the park while Steven Reid impressed at right-back and got forward to good effect.
There was a carnival atmosphere amongst Blues fans after the defeat of Arsenal, although there was very little for the 27,013 present at St Andrew's to get excited about in the first half.
Hodgson brought on Peter Odemwingie for the second period and he made an instant impact in setting up the opening goal for Mulumbu in the 47th minute.
Birmingham responded immediately and equalised within a minute through Beausejour, who nipped in between two defenders to steer home Lee Bowyer's cross.
A superb strike from Morrison restored Albion's lead in the 58th minute.
Odemwingie missed a great chance to double the visitors' advantage after Chris Brunt's shot had hit a post.
But with 18 minutes remaining Scharner did make it 3-1. Blues failed to pick up Brunt from a short Morrison corner and his centre was headed over the line by Scharner at the far post.