Historical records of mosasaur discoveries in the Hornerstown Formation of the Atlantic Coastal Plain of New Jersey (USA) are reviewed in the light of recent geochemical and biostratigraphic investigations. The highest (i.e. youngest) mosasaur remains are from the basal Hornerstown Formation fossil assemblage, a widespread concentration of vertebrate and invertebrate remains that coincides with an iridium excursion and the micropalaeontological K/Pg boundary. Most of the mosasaur specimens from this horizon are single elements, but at least one specimen from the Hornerstown Formation, YPM 773, is a multi-element specimen, a skull with vertebrae. This specimen is a very large individual of Mosasaurus hoffmanni from the West Jersey Marl Company works at Barnsboro, New Jersey. It represents the stratigraphically highest multi-element articulated specimen in the K/Pg section here. In and above the basal Hornerstown, the fauna is dominated by crocodylians, lamnid sharks and chelonians. The extinction of mosasaurs created a vacant ecological niche at the top of the marine food web, and a trophic cascade of smaller predators ensued. Later in the Neogene the appearance of Elvis taxa archaeocetes such as Basilosaurus converged on mosasaur body shape.