Albion set themselves up for a third successive win following early strikes from Thomas and Brunt.
Louis Carey then turned the ball into his own net 40 seconds into the second half before Simon Cox struck in the 48th minute - taking Albion's goal tally in their last two home games to nine - as Albion moved a point clear of Newcastle United.
All City had to show for efforts was an 83rd-minute consolation strike by Paul Hartley who struck with a 35-yard free-kick.
It may have meant little to City, but it was a special goal for Hartley - coming in his 400th league start.
Things could have been different for City, who crashed to only their third defeat of the season, if Cole Skuse had converted his chance seconds before Thomas broke the deadlock.
They ultimately paid a heavy price for Skuse squandering an excellent close-range opportunity.
City tried to battle their way back into the game before half-time and ten-goal striker Nicky Maynard was twice frustrated by Scott Carson while Hartley saw his free-kick whistle just wide.
But the damage had been done by Albion's early assault and there was no way back for Gary Johnson's side just three minutes after his half-time pep-talk.
Thomas fired Albion ahead in the third minute only seconds after they should really have fallen behind.
With Carson caught in no man's land after racing out of his goal to try and beat Maynard to the ball, Skuse fired in a shot that Gianni Zuiverloon cleared off the line.
Albion immediately surged forward with Graham Dorrans releasing Thomas. His pace took him clear of the City defence before he sent a low left-foot drive from the edge of the area just inside the near post of goalkeeper Dean Gerken.
Albion quickly doubled their advantage when Brunt collected a long throughball from Dorrans in the 12th minute.
Northern Ireland international Brunt controlled the ball on the edge of the area and then drew Gerken out of his goal before rolling his shot into an empty net from six yards.
Any hopes the Robins had of a comeback were then snuffed out seconds after the interval when Cox met a cross from Zuiverloon and saw his shot deflected home by Carey.
But Cox did not have to wait long to get his name on the score-sheet as, just two minutes later, he met a centre from Marek Cech and twisted in the area to make space for himself before pouncing from close range.