Thermal tolerance greatly influences the geographic distribution, seasonality, and feeding habits of mosquitoes; this study aimed to examine the impacts of species, sex, and diet on thermal tolerance in mosquitoes. We found that Culex quinquefasciatus was inherently significantly more cold tolerant than Aedes aegypti, while Ae. aegypti had improved heat tolerance compared to Cx. quinquefasciatus. There were no differences in thermal tolerance between sexes within either species. We observed similar levels of cold tolerance between all diets tested, but observed decreased heat tolerance in mannitol-fed mosquitoes. Our results suggest that although dietary factors such as sugar alcohols and sugars may play a role in thermal tolerance in mosquitoes, there are likely physiological and genetic factors that can have a greater influence on the limits of thermal tolerance within a species.