The Common Mudpuppy (Necturus maculosus (Rafinesque, 1818)) is found in many watersheds from eastern Canada to the southeastern United States. Although its range is large, recent observations suggest its population is dwindling in the Great Lakes region. A lack of understanding about its distribution at a finer scale or whether diet and body condition exhibit seasonal patterns limits our ability to develop a conservation management plan. This study investigated seasonal changes in Common Mudpuppy diets and body condition across western New York (USA) over a 2-yr period by using rock turning (RT) and trapping (TR) collection methods. Common Mudpuppies were found in all four major watersheds of the region in both lentic and lotic habitats. RT was more efficient than TR in streams, whereas TR was a better option in lakes. Male-to-female sex ratios and four of five measured morphological features did not differ between collection methods, although the largest and smallest Common Mudpuppies were captured by RT, suggesting some size selectivity in TR. Body girth was significantly smaller for Common Mudpuppies collected by RT, and this was attributed to seasonal differences in activity. Stomach contents were varied and differed by season, with 41 unique prey types recovered, including several forms of microplastics. In summer and fall, Common Mudpuppies fed on invertebrates exclusively, but during winter and spring, fish were incorporated into the diet. Body condition reflected the change in diet, with a higher body condition when fish were present in the diet. These findings suggest seasonal data are necessary to fully understand Common Mudpuppy conservation needs.