Paul Robinson and Bednar celebrate West Brom's victory
Roman Bednar's injury-time winner increased the pressure on Manchester City boss Mark Hughes, whose side remain in the Premier League drop zone.
An open first half saw Bednar hit the post before Luke Moore ran on to James Morrison's pass to put bottom side West Brom ahead after 69 minutes.
City replied when Felipe Caicedo's deflected backheel hit the post and bounced over the line off Scott Carson.
But Bednar won it when he rose unmarked to head home Gianni Zuiverloon's cross.
Saturday's results had left City in the bottom three for the first time this season and their recent slide down the table has inevitably raised question marks over Hughes' future, despite protestations to the contrary from the powers that be at Eastlands.
The club's new oil-rich Arab owners are likely to invest millions in strengthening the City squad in January but, with just one win in their last nine Premier League games, many fans are unconvinced that their current manager should still be in charge of the spending when the transfer window reopens.
With a festive programme that includes a home game against high-flying Hull and a trip to a rejuvenated Blackburn, it was vital for the Welshman that he came away from the Hawthorns with a positive result.
But, with the Baggies just as desperate for points, achieving that was never going to be straightforward; especially given City's appalling domestic away record this term.
Given this was a relegation battle, it was unsurprising that the game began at a frantic pace with clear-cut chances hard to come by.
Both sides have struggled for goals of late - managing two each in their previous five games - and it was easy to see why as they failed to capitalise on some promising situations.
City's strikers Darius Vassell and Benjani seemed out of tune with the rest of their side and the visitors' best moments going forward were through Stephen Ireland and Shaun Wright-Phillips, who had two efforts deflected wide.
Mowbray lauds West Brom belief
West Brom's frontmen, Moore and Bednar, were showing plenty of willing but little quality until Bednar collected a pass from Paul Robinson and smashed a shot against the inside of Joe Hart's post.
That was the highlight of a first half that ended when Benjani fired wildly over from six yards after taking the ball off Micah Richards' toe.
There continued to be a lack of quality on display in the second half, with City carving out the best opportunity when Pablo Zabaleta crossed for Wright-Phillips to fire a snap-shot straight at Carson.
But, out of nothing, the Baggies took the lead on a swift break from a City corner when Morrison played in Moore, who coolly rolled the ball home for his first goal for the club.
City's response was to pour forward and, with seven minutes left, they must have thought they had secured at least a point when Caicedo's clever backheel came off Chris Brunt, clipped the post and bounced over the line off Carson.
Hughes despairs at 'fundamental errors'
The visitors went for the win and paid the price when, from another corner, Abdoulaye Meite made good ground down the right and Zuiverloon crossed for Bednar to nod home a dramatic winner .
Richards allowed the Czech striker to rise unchallenged to head home, thinking Hart was coming to claim the ball, but the end result was City's 10th league defeat in 18 outings and, following last weekend's defeat by Everton, the second time in eight days they have been beaten in injury-time goal.
West Brom are still a point adrift at the foot of the table and four points from safety, but they have been bottom of the Premier League at Christmas before and stayed up - and this win will give them hope that they can repeat the trick this season.
West Brom: Carson, Zuiverloon, Meite, Olsson, Robinson, Morrison, Koren, Greening (Brunt 83), Kim (Dorrans 64), Bednar, Moore (Beattie 73). Subs Not Used: Kiely, Hoefkens, Barnett, Pele.
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