Human and ape mandibles differ in the proportion of adult size attained at equivalent dental emergence stages; for most dimensions human mandibles are more advanced. These dissimilarities in pattern of growth underlie the vastly different adult mandibular morphologies of these taxa. Australopithecus mandibles represent a third distinctive mandibular morphology, but the pattern of its mandibular growth remains underexplored. The Australopithecus afarensis sample from the Hadar site, Ethiopia, ca. 3.4–3.0 Ma, is represented by three infant (pre-M1 emergence) and two juvenile (pre-M3 emergence) mandibles. A recently recovered mandible, A.L. 1920-1, though edentulous, appears to capture an A. afarensis individual during M2 emergence, thus bridging these developmental stages. In this chapter, we (1) describe three new infant/juvenile A. afarensis mandibles and confirm that the suite of features used to distinguish A. afarensis from other taxa is present early in ontogeny, and (2) investigate how the A. afarensis mandible changes in size and shape throughout growth in comparison to humans and chimpanzees. Our results indicate that A. afarensis resembles humans more than chimpanzees in its percentage of adult corpus breadth attained at successive stages of dental emergence. A. afarensis is also more similar to humans in corpus cross-sectional shape changes throughout ontogeny. We suggest that canine reduction may have had an important influence on the growth trajectory of the A. afarensis mandibular corpus such that, as in humans, it achieved adult values relatively early. Our results underscore the importance of considering the influence of the developing dentition on both juvenile and adult mandibular morphology.