Danny Wilson was the manager who took Barnsley into the Premiership seven years ago, but he was back on his old Yorkshire stomping ground hoping to put one over on his former club and steer City to three valuable points.
And the visitors secured a worthy win thanks to Tony Rougier's splendid first-half goal.
It could have been more, with City spurning at least three other excellent chances while Barnsley looked far too lightweight up front and appear to have a tough job ahead in the summer to rebuild for next season.
The Robins bagged the lead with their first attempt on goal after 21 minutes.
Barnsley had forced a corner but the visiting defence cleared the danger and Rougier broke away down the left and skipped all too easily past Antony Kay before drilling a low shot past on-loan goalkeeper Gavin Ward.
Seven minutes later Christian Roberts got clean through down the left but was thwarted by Ward's brave block.
Just before the break a dreadful error by Ward presented Lee Peacock with a clear opportunity to make it 2-0 but the City striker lost his cool and blasted the ball high and wide.
Seconds later impressive visiting captain Tom Doherty went past his counterpart Craig Ireland but dragged his shot wide of the far post.
The second half was rather a nondescript affair, with the home side, having nothing to play for but pride, appearing to go through the motions.
City had other chances, particularly through striker Lee Peacock, but they had to settle for a one-goal success.
They now go into the last match of the season a point behind second-placed QPR.