The thermoregulation of mosasaurs and plesiosaurs have been debated within the field of paleontology for many years, specifically whether these marine reptiles were ectothermic or endothermic. Some degree of endothermy has been proposed for both mosasaurs and plesiosaurs based on histological, morphological, and geochemical methods. Geochemical studies have suggested elevated body temperature in mosasaurs and plesiosaurs compared to ambient seawater temperatures, but the topic remains understudied. For this study, bulk oxygen isotope paleothermometry was used to calculate the body temperatures for Late Cretaceous mosasaurs and plesiosaurs from the Pembina Member of the Pierre Shale in south-central Manitoba. Body temperatures were also calculated for coeval fish and birds (Hesperornis) from the Pembina Member which were used as ectothermic and endothermic ‘end members’, respectively. Before temperatures were calculated, the preservation of 25 fossils specimens were assessed using scanning electron microscope analysis, relative total rare earth element concentrations, principal component analysis of bulk element concentrations, and the difference between the δ18O in the phosphate and carbonate fractions. Using bulk oxygen isotope palaeothermometry, body temperatures were calculated to be 36.4 ± 1.1°C SD for mosasaurs, 42.8 ± 0.3°C for plesiosaurs, 28.1°C for fish and 41.1°C for Hesperornis. Statistical analysis could not be conducted due to the small sample size, but the body temperatures of Manitoba mosasaurs did fall between coeval ectothermic fish and endothermic birds and were consistent with those calculated from other localities. Plesiosaur specimens also possessed elevated body temperatures but exceeded those calculated for both fish and birds as well as those previously determined for plesiosaurs from different localities. The elevated temperatures calculated for the Manitoba plesiosaur specimens may reflect differences in body temperatures between genera or may be the result of diagenetic alteration. The body temperatures calculated for the marine reptiles in this study are elevated compared to ambient seawater temperatures and thus, can be taken as evidence of endothermy in Late Cretaceous Manitoba mosasaurs and plesiosaurs.