Goals either side of half-time from Jeff Hendrick and Jamie Ward, who saw a third-minute penalty saved by Tom Heaton, and an injury-time strike from substitute Ben Davies leaves the Robins rooted to the bottom of the table.
The margin of defeat would have been greater had it not been for Heaton, the woodwork, and a contender for miss of the season from Derby striker Conor Sammon.
Defeat at Pride Park was a seventh in a winless run of nine on the road and a 14th away league loss of the season for City.
Derby had the chance to make the perfect start when Hendrick's surging run from midfield was unfairly halted by Greg Cunningham. Ward stepped up to take the resulting penalty but his effort was easily saved by Heaton who, diving to his left, gathered the Northern Ireland international's side-footed shot in his midriff.
The Rams had another golden opportunity to take the lead after 10 minutes when Craig Bryson, wrongly shown a yellow card for diving after only 49 seconds, beat Bristol City's defenders and goalkeeper to a long ball forward. The midfielder then won the race with Lewin Nyatanga to the loose ball inside the six-yard box and looked certain to score but the former Derby centre-half made a last-ditch sliding tackle to block Bryson's shot at the expense of only a corner.
Derby deservedly went ahead after 36 minutes. Ward's corner was volleyed away at the near post by the third former Ram in the Robins squad - Steven Davies. But Hendrick, stationary just outside the box, volleyed the ball straight back and his steered side-footer eluded everyone inside a crowded penalty area and bounced past Heaton into the bottom left corner of the net.
The Rams doubled their lead nine minutes into the second half, with Ward making amends for his penalty miss. Sammon got free down the right and switched the play across the Bristol box to Ward, who got the ball under control with his right foot before blasting a left-footed shot beneath Heaton from inside the area.
Sammon should have made it 3-0 in the 78th minute from Chris Martin's low cross but somehow managed to scoop the ball over the crossbar from four yards out when totally unmarked.
Ben Davies capped successive league wins for Derby for the first time since mid-December, when the Rams beat their opponents 2-0 in the reverse fixture at Ashton Gate, with a low shot across Heaton from a tight angle down the left side of the box.