Barnsley recorded a second victory from two group fixtures in this season's EFL Trophy, seeing off City at Oakwell.
First half headed goals from Conor McCarthy and Jon Russell were followed in the second half when debutant Emmaisa Nzondo grabbed a third.
There was an early sighter for Fabio Jalo when Max Watters cleverley chested a diagonal ball into his path, but the 17-year-old pulled his shot wide from the edge of the box.
Young wing-back Hayden Pickard did well to work some room inside the box, left-hand side before releasing Watters, but Max drove his strike just over the bar from 16 yards.
Jalo was then denied when racing through one-on-one, with a defender making a last-ditch block to thwart his attempt at goal.
The breakthrough arrived on 26 minutes when Josh Benson's inswinging corner kick was nodded home inside the six yard box by captain, Conor McCarthy.
Jon Russell doubled the lead soon after when he scored with his own header from a corner, meeting Pickard's delivery and seeing his effort crash in off the bar.
But the visitors responded before the interval as Will Dickson slotted home from two yards out after a lightning raid down the left flank.
HALF TIME: Barnsley 2-1 Man City
The second 45 was much quieter for the most part, but Jalo nearly added a third when Josh Benson found him inside the box but the Portugal U19 international's wicked shot flew just a few inches wide.
Jon Bland and Nzondo came on for their debuts midway through the half, and Emmaisa sealed the victory when his looping header on the end of Theo Chapman's cross evaded the outstretched hand of the City stopper.
There was another debutant in the dying stages when Mylan Benjamin replaced Chapman on what was a confident performance from a much-changed side.
We've to wait now until November for our final group F fixture against Bradford City at Valley Parade.
FULL TIME: Barnsley 3-1 Man City
Killip, D.Benson (Bland), McCarthy, de Gevigney, Shepherd (James), Pickard, Russell (Yoganathan), J.Benson, Chapman (Benjamin), Watters (Nzondo), Jalo.
Unused: Flavell, Dyer.
