Significance The neuromuscular junction (NMJ) is a synapse between the motor nerve and myotube essential for controlling skeletal muscle contraction. Motor nerve-derived glycoprotein agrin is indispensable for the formation and maintenance of NMJs, and genetic defects in agrin underlie a congenital myasthenic syndrome (CMS). Agrin’s role has been thought to be activation of the muscle-specific receptor kinase MuSK. Here, we demonstrate that forced activation of MuSK in agrin-deficient mice restored embryonic formation, but not postnatal maintenance, of NMJs, demonstrating that agrin plays an essential role distinct from MuSK activation in the postnatal maintenance of NMJs. Given that CMSs frequently show postnatal onset, this finding provides key insights not only into NMJ homeostasis but also into CMS pathology with unknown etiology.