The sentence in question is "His first two clients were the last two persons hanged in the Maycomb County jail." This leads us to think that he did not succeed in defending them, and that he was not a particularly good attorney. However, we learn he really didn't have much of a chance to defend them because they committed their crime in "the presence of three witnesses" and then were foolish enough to plead "not guilty." Atticus "was present at their departure," meaning he saw them hanged, which is why he now has a "profound distaste for the practice of criminal law." We learn in the story, however, that Atticus is a very accomplished lawyer, and that no one but he could have made the jury deliberate about Tom as long as they did.