The former Everton star grabbed the vital 59th minute goal to relieve the growing pressure at St. Andrew's to give Birmingham their first home win since they beat Watford on December 6.
It was a battling performance by Birmingham following their recent dismal run of only one win in six games.
The victory, watched by their lowest crowd of the season keeps them in the promotion race just as couple of points behind second-placed Reading.
Meanwhile, Derby are still looking for their first win under the management of Nigel Clough.
A much changed Birmingham side were more than a little anxious in the early stages.
Goalkeeper Maik Taylor fumbled a cross in the first minute but then restored his confidence with two smart saves from Gary Teale.
Taylor had to be at his best in the 19th minute when the recalled Martin Taylor made a bad mistake to let in Rob Hulse but the Birmingham goalkeeper made a fine diving save to his left to prevent County striker opening his account.
Derby's shoot on sight policy certainly had Birmingham on the back foot and raised doubts about the lack of midfield control by the home team.
Subsequently Birmingham attempted to get back into the game but they appeared badly disjointed at this stage.
Savage continued to make a big impact on his return to St. Andrew's where he claimed he had previously enjoyed the best playing spell of his career.
His industry enabled Derby to keep moving forward and as a result they were undoubtedly the more dangerous than their opponents until Birmingham lifted their game going into the second half.
Only a timely interception by Kris Commons prevented Kevin Phillips gaining possession but from the resultant corner, the ball was played back to Hameur Bouazza and his cross was headed home by Carsley for only his second goal for Birmingham.
Derby, with only one win in their previous ten games, gallantly attempted to salvage a point but a reflex fingertip save by Taylor from Commons' header denied them a share of the spoils.
Carsley then made a goalline clearance from a Miles Addison effort to demonstrate his leadership qualities.