The UDP-glucuronosyltransferase (UGT) gene family is responsible for the transfer of glucuronic acid to exogenous and endogenous chemicals. Based on the highly diversified number of genes, the mammalian UGT1A and UGT2B subfamily genes are believed to be involved in the conjugation reactions of xenobiotic metabolism. However, it is speculated that the UGT2 family genes are not involved in the xenobiotic metabolism of avian species due to the less diverse number of genes. In this study, we aimed to investigate the evolutionary history of mammalian UGT1 and UGT2 family genes and determine when the diversification of UGT2B genes occurred. We also attempted to identify the main factors responsible for the diversification of UGT genes. By examining the genomic information and feeding habits of 67 species representing each mammalian family, we discovered that the UGT2B genes emerged in the Eutheria on or after Cretaceous period and that their number were higher in plant-eating mammals (herbivore or omnivore) than in carnivorous mammals. We also found that the UGT2B genes in some herbivorous mammals underwent positive selection. In contrast, the diversity of the UGT1 family genes was inherited from the common ancestor of birds and mammals. Thus, our findings suggest that the emergence of angiosperms (flowering plants) and the occurrence of “animal–plant warfare” influenced the evolution of this gene family involved in the xenobiotic metabolism of eutherians. Furthermore, future research investigating the marsupials and birds that do not possess UGT2B genes is required to elucidate the mechanisms underlying the metabolism of chemical substances in these species.