But the game was over as a contest within three minutes of the restart as Albion swept into a 4-0 lead courtesy of Louis Carey's own goal and Simon Cox's second league strike of the season.
City deservedly pulled one back late on through Paul Hartley's superb free-kick but it was nothing more than a consolation.
The result only confirmed Albion's superiority over their promotion rivals as they continued where they left off prior to the international break, and they now top the table, at least until Monday night, when Newcastle travel to Preston.
For Bristol City, a side who had suffered the fewest defeats in the division prior to kick-off, it must surely go down as 'a bad day at the office' as some naive defending and wasteful finishing ultimately proved their undoing.
Wintery conditions greeted both sides in the Black Country with rain and wind swirling around The Hawthorns but a crowd of 23,444 were kept warm by an entertaining opening.
Having seen Gianni Zuiverloon clear Cole Skuse's effort off the line in the third minute, Albion fans watched their side counter at speed and open the scoring at the other just 30 seconds later.
Thomas rounded off a sweeping move as he cut in from the left before firing a fierce shot into the bottom corner from the edge of the penalty area.
Scott Carson was forced to tip Carey's header over the crossbar before Brunt doubled West Brom's lead with just 12 minutes gone.
Scotland midfielder Graham Dorrans found his Northern Irish team-mate with a great throughball and he rounded Dean Gerken before firing home from an acute angle to put the home side in control.
But Carson was soon called upon to deny the lively Nicky Maynard before Dorrans himself side-footed wide from close range when he really should have put the result beyond doubt.
Chances were coming at either end as City midfielder Hartley then lashed a free-kick inches wide and, with the visitors more than holding their own, the difference at the half-time interval proved to be Albion's ability to take their chances.
But if the game was still somewhat in the balance at the break then it was all over just two minutes after the restart.
Cox picked up the ball and sent a shot goalwards, which appeared to be going wide, until Carey stepped in to intercept and could only fire the ball into his own net.
Albion were threatening a rout as they made it 4-0 just 53 seconds later - Cox sliding in to convert Marek Cech's cross from close range and put his name on the scoresheet.
From then on in it was a case of going through the motions for Albion, with Zuiverloon failing to pull the trigger when in a great position to make it 5-0 while Danny Haynes wasted a good chance at the other end.
But the Robins did grab a consolation in the 83rd minute through Hartley's stunning set-piece.