Plesiotylosaurus was a relatively large mosasaur. The holotype, LACM 2759, consists of a poorly preserved and partially distorted yet almost complete skull and mandibles collected from the Panoche Hills in the Moreno Formation, California. The skull measures 90 centimeters in length, which would give the whole animal an estimated size of about 6,5 meters in length. The jaws of Plesiotylosaurus were elongated and its skull was quite robust, similar to Prognathodon, which suggests a distinct prey preference.