McCarthy's men failed to trouble Paddy Kenny until the closing stages, when the Leeds goalkeeper made instinctive saves from Lee Martin and substitute Michael Chopra.
A goal in each half by Jerome Thomas and Paul Green had earlier given the hosts a two-goal advantage as the Tractor Boys saw their three-match winning run splutter to a halt.
McCarthy said: "We had our chances, even at 2-0, and if we had scored it would have been a nervous end for them. We have made progress as a team and had a really good run, so now we have to start again.
"We gave a bad goal away in the first half when we didn't defend a corner correctly and then we got done on the break, but it was a fabulous finish by their player.
"We missed our chances and that was the difference. We were on top in the second half, only to let them off the hook, so I am disappointed."