β-lactam antibiotics inhibit penicillin binding proteins (PBPs) involved in peptidoglycan synthesis. Although inhibition of peptidoglycan biosynthesis is generally thought to induce cell lysis, the pattern and mechanism of cell lysis can vary substantially. β-lactams that inhibit FtsI, the only division specific PBP, block cell division and result in growth as filaments. These filaments ultimately lyse through a poorly understood mechanism. Here we find that one such β-lactam, cephalexin, can, under certain conditions, lead instead to rapid lysis at nascent division sites through a process that requires the complete and ordered assembly of the divisome, the essential machinery involved in cell division. We propose that this assembly process (in which the localization of cell wall hydrolases depends on properly targeted FtsN, which in turn depends on the presence of FtsI) ensures that the biosynthetic machinery to form new septa is in place before the machinery to degrade septated daughter cells is enabled. β-lactams that target FtsI subvert this mechanism by inhibiting FtsI without perturbing the normal assembly of the cell division machinery and the consequent activation of cell wall hydrolases. One seemingly paradoxical implication of our results is that β-lactam therapy may be improved by promoting active cell division.