Town and Birmingham are level at 1-1 at half-time, David Cotterill having given the visitors the lead in the 22nd minute and Brett Pitman equalising for the Blues 10 minutes later.
Winger Larsen Touré was handed his full league debut with Ryan Fraser, dropping to the bench having ben a doubt after taking a kick in Tuesday’s 1-0 win at Leeds.
Up front, McCarthy started with Pitman and Freddie Sears with Daryl Murphy and David McGoldrick on the bench.
Birmingham made three changes, Andrew Shinnie, Neal Eardley and Demarai Gray coming in for Jon Toral, Paul Caddis and Jacques Maghoma.
The Blues started on the front foot and in the third minute Pitman hit a low 25-yard shot which Birmingham keeper Tomasz Kuszczak claimed down to his right.
On six Touré won a corner on the right, Ainsley Maitland-Niles sending over the flag-kick and a defender diverting it over ahead of Tommy Smith.
A minute later, Maitland-Niles cut the ball back from the left of the area to Sears, who shot only just over Kuszczak’s goal.
The Blues continued to press, Pitman heading a looping 10th minute right-wing Touré cross wide.
David Cotterill picked up the game’s first yellow card for a late challenge on Touré with Town on the break after the former Bristol City man had lost possession to Maitland-Niles on halfway.
Birmingham threatened for the first time in the 13th minute but Town skipper Luke Chambers blocked Shinnie’s powerful shot on the edge of the box.
Moments later, left-back Jonathan Grounds exchanged passes with Shinnie and broke into the area on the left but Dean Gerken was off his line quickly to block. Play continued and Cotterill chipped weakly into Gerken’s arms.
The Birmingham winger hit a subsequent shot well wide on 16, then two minutes later Touré was even more wayward at the other end, his strike flying deep into the lower tier of the Sir Alf Ramsey Stand.
Town had begun the game strongly but the visitors had started to look the more dangerous side and in the 22nd minute they went in front, although with some element of good fortune.
Shinnie latched onto a flick-on from a throw on the left and, as Christophe Berra lost his footing on the wet pitch, broke into the box before finding Cotterill breaking into the area on the right, and the Wales international smashed the ball past Gerken.
However, the lead would last only 10 minutes with the Blues getting back on terms in the 32nd minute from the penalty spot.
Maitland-Niles made a superb run from inside his half and just about reached the ball on the byline and cut it across but into Kuszczak’s arms. As he crossed Jonathan Spector dived in and, after the ball had gone, took the on-loan Arsenal man’s legs from under him.
Despite there being few appeals from Town players, referee Keith Stroud pointed straight to the spot, much the anger of the Birmingham players who protested bitterly. Pitman took the spot-kick and confidently beat Kuszczak to net his third goal of the season.
Cotterill forced Gerken into an uncomfortable save in the 34th minute after a freekick had been harshly awarded against Jonathan Douglas on the left, five yards outside the area. The Birmingham winger’s shot moved wickedly in the air and Gerken only just pawed it away from goal.
Town claimed another penalty a minute later when Sears went to ground under pressure from Eardley but the Town striker appeared to have been fairly outmuscled and the initial challenge looked to be outside the area in any case.
Cotterill continued to try his luck from distance, shooting wide in the 41st minute, prior to Maitland-Niles twisting and turning five yards outside the Birmingham box and hitting a shot well over.
As the half moved into injury time Paul Caddis replaced the injured Eardley for the visitors.
Neither side threatened again before the whistle ended an entertaining and open half with the scoreline a fair reflection of the game.
The Blues started well but failed to make their early dominance tell. The visitors began to get more into it and Grounds had had the game’s best chance prior to Cotterill’s goal, which owed much to Berra’s slip.
Town hadn’t looked particularly like getting on terms until the penalty, which was the result of a tremendous Maitland-Niles run.
While Spector certainly arrived late and caught the Blues winger, it was the type of foul which is usually given outside the area but less often in the box. Pitman took his penalty with great composure to leave the scores level at the break.
Town: Gerken, Chambers (c), Smith, Berra, Knudsen, Skuse, Douglas, Touré, Maitland-Niles, Pitman, Sears. Subs: Bialkowski, Malarczyk, Parr, Coke, Fraser, McGoldrick, Murphy.
Birmingham: Kuszczak, Eardley (Caddis 45), Spector, Morrison (c), Grounds, Gleeson, Kieftenbeld, Cotterill, Shinnie, Gray, Donaldson. Subs: Legzdins, Robinson, Maghoma, Toral, Davis, Brock-Madsen. Referee: Keith Stroud (Hampshire).