After being completely outplayed in the first half, self-belief was sparked by the gift of a goal early in the second for ex-Doncaster Rover Paul Green to tuck away.
At the end only two tremendous saves by Chris Weale from Emanuel Villa came between Derby and a win.
Manager Paul Jewell might have signed 13 players since Derby fell through the Premier League trapdoor, but his problems in defence are still stark.
City got their goal through Nicky Maynard - their £2.25million summer buy from Crewe, midway through the first period.
It was a tap in after a whole-hearted run by Brian Wilson, an exchange with Gavin Williams and a Wilson ball in which caught the visitors cold.
Wison had come on eight minutes earlier after Jamie McCombe had been carried off with a broken cheekbone.
That injury caused problems for manager Gary Johnson who didn't have a central defender on the bench, but Wilson came on at right-back to win the man-of-the-match award.
Derby had only one player who looked capable of hurting City in Kris Commons on the left flank.
His crosses deserved better that they got from Derby and he also delivered their only two worthwhile shots of the first half.
Only sharp saves by Roy Carroll kept Derby in the game, but then they had a piece of good luck which has been in very short supply.
Inside his own box, City's captain Lee Johnson sent a short defensive pass straight to Green who stunned City with his cool finish.
For the first time the visitors looked sharp and aggressive and, as Bristol's game fell away, Nathan Ellington crashed in two shots just wide of target.