Birmingham manager Alex McLeish signed a new three-year contract - then saw his side overwhelmed by the Baggies.
Blues controlled the first half and deservedly led at the interval through Cameron Jerome's first goal of the campaign.
But the Baggies were transformed after the break with an own goal from Scott Dann and goals from Peter Odemwingie and Jonas Olsson dooming City to their first defeat of the season.
Odemwingie caused all sort of problems when pushed into attack by Albion boss Roberto Di Matteo for the second half in place of Marc-Antoine Fortune, while Jerome Thomas tormented the Blues defence throughout the 90 minutes with his trickery and powerful runs and testing crosses.
Alexander Hleb made a promising debut for City on loan from Barcelona but, like his team-mates, had to play second fiddle to the home side during the second period.
Odemwingie should have put the Baggies ahead after 14 minutes.
Thomas made a decisive run into the Birmingham box before turning the ball back across to the unmarked Odemwingie.
But he failed to make proper contact on his shot and Ben Foster was able to make a comfortable save to his right.
It proved a costly miss and 60 seconds later Jerome put unbeaten Blues in front with his first goal of the season.
Larsson floated a free-kick from the central part of the pitch to Ridgewell who turned the ball back across goal - and Jerome made no mistake from close range.
West Brom tried to retaliate and Foster blocked a shot from a narrow angle by Paul Scharner following a deep centre by Brunt.
Another probing Thomas centre found Brunt but his header did not over extend Foster.
At the other end Gardner got plenty of power into his first-time 25-yard drive but Scott Carson had his body behind the ball.
Gardner then produced a vital interception inside his own six-yard box to cut out a cross from Brunt with Scharner unmarked behind him and with the goal at his mercy.
Albion finished the first half with a flourish and Gabriel Tamas powered a header just past the post from a deep centre by Brunt.
Di Matteo made a half-time substitution with midfielder Graham Dorrans replacing Fortune, whose place in attack was taken by Odemwingie.
After 51 minutes an own goal from Dann brought Albion back on level terms.
Thomas was the creator with a run to the by-line and he held off the challenge of Stephen Carr before supplying a low cross.
Dann, under pressure from Odemwingie, could only deflect the ball past Foster.
Blues tried to retaliate and Gardner's low cross-shot was only just off target while Jerome's 20-yard strike was just too high.
But after 59 minutes Albion went ahead through Odemwingie.
Bowyer tried to cut out a ball across the box but his miscontrol let in Odemwingie who calmly took the ball wide of Foster before slipping into the net.
Odemwingie was yellow carded for his goal celebrations.
Bowyer needed lengthy treatment after being on the receiving end of a heavy challenge from Jara.
The City midfielder appeared to be losing his cool and was yellow carded after fouling Tamas. He was immediately substituted and replaced by Nikola Zigic.
Bowyer was involved in some exchanges with Albion fans sat behind the Blues dug-out area and eventually moved seats.
Then after 69 minutes Olsson increased Albion's lead when he was unmarked when heading home an inswinging corner from Brunt.
Carson handed referee Chris Foy an object which was thrown on the pitch from the area housing Birmingham fans behind his goal.
Brunt was too high with a 25-yard effort but Albion were now in complete command.
Thomas curled a 20-yard shot just wide before leaving the pitch to a standing ovation and being replaced by Giles Barnes.