We present the first detailed account of cranial ontogeny in a fossil marsupial. An exceptionally well-preserved ontogenetic sample representing 26 individuals of Nimbadon lavarackorum (Diprotodontidae, Zygomaturinae) is described from a 15 million-year-old cave deposit in the Riversleigh World Heritage Area, Queensland. Based on comparison with one of N. lavarackorum's closest living relatives, the common wombat (Vombatus ursinus), the N. lavarackorum sample represents developmental stages spanning suckling pouch young through to elderly adults. In addition to documenting ontogenetic changes, reduced major axis regression analyses of 14 cranial and seven mandibular measurements were used to analyze allometric growth. Early growth patterns in the cranium are comparable to those of other marsupials such that the bones of the facial portion of the cranium develop precociously compared with those of the neurocranium. Most differences between adult and juvenile crania result from reorganization of structures associated with the cranio-mandibular joint and the developing dentition and strengthening of areas for insertion of the muscles of mastication. These changes reflect the transition from a purely suckling function to one focused on mastication of leaves. The sample also provides a unique view into the development of the extraordinarily complex endocranial sinuses characteristic of diprotodontid marsupials.