McCarthy only took the reins at Town on Thursday morning but his impact has been instant, with the Suffolk outfit claiming only their second league win of the season and first since late August, although they still remain rooted to the foot of the Championship.
It could and should have been a point apiece but City substitute Nikola Zigic wasted two golden second-half chances.
After a distinctly ordinary opening to proceedings it was Ipswich who drew first blood with just eight minutes on the clock.
Nathan Redmond gave possession away cheaply in the middle of the park and Lee Martin's effort fell to one-time Birmingham striker DJ Campbell, who turned and dispatched a fine shot across Jack Butland for his third goal in five appearances since joining on loan from QPR.
Daryl Murphy tested Butland with a low effort before Marlon King forced Stephen Henderson to tip over his goalbound 25-yard free-kick at the other end.
Martin was next to try his luck from distance with a shot which Butland turned behind before the first half fizzled out, with Birmingham booed off at the break.
Danny Higginbotham's header then brought a good save from Butland after Blues had switched off from Ipswich's short set-piece.
Wade Elliott blasted wide for the home side when well placed before Zigic, in space, somehow headed wide from King's inch-perfect cross when he really should have scored.
The towering Serbian striker wasted another great opportunity moments later when he fired straight at Henderson when clean through.
Ravel Morrison's deft chip found the roof of the net before Curtis Davies blasted over in injury time as Blues continued to push for an equaliser, but Ipswich held firm to claim their first three points at St Andrew's since 1986.