Long-term monitoring observations have revealed the characteristics of species diversity formation and population dynamics of small mammals in an abandoned meadow undergoing natural succession. The early stages of meadow succession resemble those of regenerating clear-cuts, characterized by high small mammal abundance and dominance of open-habitat species. Due to the specific vegetation changes in the studied meadow—particularly the formation of a cow parsley plant association in later successional stages – habitat conditions became less favorable for small mammals. This led to gradual changes in key population characteristics of rodents and insectivores. In the later stages of meadow overgrowth, both species diversity and the abundance of individual species declined. Different small mammal species responded differently to these changes. The transformation of meadow vegetation had no significant effect on the common shrew, whose population dynamics followed the general population trends in the study area. In contrast, the bank vole and the root vole, which depend on the availability of food plants, experienced a decline in abundance as the meadow overgrew with cow parsley. Other small mammal species used the meadow as a suboptimal habitat during dispersal of yearlings and population peaks. Since the formation of cow parsley associations is characteristic of abandoned agricultural lands across much of our region, further research is needed to understand the population dynamics of small mammals in such environments.