Like other mosasaurs, Platecarpus had a long, laterally flattened tail, steering flippers, and deadly, tooth-lined jaws. It was around 4.3 meters (14 ft) long, with half of that length being taken up by its sinuous tail. It probably swam in a snake-like fashion. Platecarpus probably fed on fish, squid, and ammonites. They were medium sized animals, reaching about 7 meters (23 ft) in length. The platecarpine mosasaurs had evolved into the very specialized plioplatecarpine group by the end of the Cretaceous.
The skull structure of Platecarpus is unique among mosasaurs. This genus is characterized by a short skull, and have the least number of teeth in its jaw than compared to any other mosasaur(approximately 10 teeth in each dentary).
Platecarpus was probably the most common genus of mosasaur in the Western Interior Sea during the deposition of the Smoky Hill Chalk in Kansas, and Platecarpus ictericus is the most commonly occuring species. There is some controversy regarding the description of the genus Platecarpus since it includes some diverse, and possibly unrelated forms.
The type specimen of Platecarpus (P. planiforms) was discovered by Professor B. F. Mudge and was classified by Edward Drinker Cope as Clidastes planiformes. In 1898, on further analysis of the remains, it was determined that the mosasaur be placed in a separate genus, Platecarpus. The type specimen underwent another taxonomic review in 1967, when paleontologist Dale Russell determined that the remains were too fragmentary to be placed within any genus, and deemed it to be a specimen of "uncertain taxonomic position". A 2006 discovery in the Smoky Hill Chalk of Kansas re-affirmed this position with the discovery a complete fossilized skull being unearthed.