Despite the fact that genes and the environment are known to play a central role in islet function, our knowledge of how these parameters interact to modulate insulin secretory function remains relatively poor. Presently, we performed ex vivo glucose-stimulated insulin secretion and insulin content assays in islets of 213 mice from 13 inbred mouse strains on chow, western diet, and ketogenic diet. Strikingly, among these 13 strains, islets from the commonly used C57BL/6J mouse strain were the least glucose responsive. Using matched metabolic phenotyping data, we performed correlation analyses of inter-islet parameters and found a positive correlation between basal and glucose-stimulated insulin secretion, but no relationship between insulin secretion and insulin content. Using in vivo metabolic measures, we found that metabolic health determines the relationship between ex vivo islet insulin secretion and fasting plasma insulin. Finally, we showed that islet glucose-stimulated insulin secretion decreased with ketogenic diet in almost all strains, concomitant with broader phenotypic changes, such as increased adiposity and glucose intolerance. This is an important finding as it should caution against the application of the ketogenic diet for beta-cell health. Together these data offer key insights into the intersection of diet and genetic background on islet function and whole-body metabolism.