Currently known from the early Campanian to late Maastrichtian, Prognathodon constitutes a group of mosasaurine mosasaurs that typically possessed massive jaws. Only four articulated skulls are known to date and the information concerning their postcrania remains minimal. Moreover, the early to middle Campanian specimens of the taxon are extremely rare and fragmentary. Two new specimens of Prognathodon overtoni from the early late Campanian (ca. 74.5 Ma) Bearpaw Formation exposed near Welling, southern Alberta, Canada, provide the first fully articulated skeletons of the genus. Detailed morphological assessment of these and pre-existing specimens indicates that many cranial characters are shared between P. overtoni and the generic type P. solvayi from the early Maastrichtian, including (1) short premaxillary-maxillary suture; (2) short triangular frontal with marked anterior constriction; and (3) distinct, blind-ended median opening on medullary floor of basioccipital. These are interpreted as generically diagnostic characters, and the first character in particular distinguishes Prognathodon from Liodon and other long-snouted mosasaurines. The postcranium of P. overtoni is lightly built, and differs very little from that of Clidastes, a basal mosasaurine. Marginal teeth are carinate with fine crenulations, indicating their proposed opportunistic predatory behavior. Additionally, most of the fully erupted teeth exhibit a similar degree of apical wear, which is interpreted as indicating that the taxon regularly handled prey items that involved crunching. To augment these dietary inferences, gut contents from one of the new specimens include fragments of a large and a small fish, a sea turtle, and possibly a cephalopod.