New data regarding the placement of the Cretaceous-Paleogene (K/Pg) boundary in New Jersey is presented based on a recent campaign of drill coring boundary sections in the Atlantic coastal plain of southern New Jersey. The occurrence of late Maastrichtian mosasaurs worldwide and in New Jersey is reviewed in light of the continuing controversy over the rate and cause of the extinction at the K/Pg boundary. At the Meirs Farm site in Monmouth County, NJ, the biostratigraphic position of mosasaur specimens (Halisaurus platyspondylus, Mosasaurus hoffmann) is related to the occurrence of an iridium excursion of 0.5 ppb (5x background levels) in the basal Hornerstown Formation just above the upper Maastrichtian New Egypt Formation. Other specimens in museum collections obtained during the acme of nineteenth century marl mining are from this area of the Maastrichtian outcrop belt in New Jersey. It is concluded that late Maastrichtian mosasaurs show no diminution of their biogeographic ranges and are not particularly rare in New Jersey in comparison to older mosasaur faunas. Mosasaurs became extinct in association with the collapse of the marine food web at the K/Pg boundary, and were replaced as apical marine predators in the early Danian by a variety of crocodilians.