Wilhelm Blandowski was the first zoologist employed by the Victorian Government, being appointed Officer of the Museum of Natural History by Governor La Trobe on 1 April 1854. Although he remained in this position for less than four years he left an important legacy by beginning the documentation of Victoria’s mammalian fauna before the full impact of European pastoralism and feral animals had become apparent. In particular, the 1856-57 zoological survey expedition to the lower Murray-Darling region provided a unique insight into the mammalian community that existed there before European occupation triggered a sudden decline in mammal species diversity, as happened progressively across the southern two thirds of Australia over the subsequent 90 years. Of the 34 mammal taxa recorded by the Blandowski Expedition, ten are extinct, nine no longer occur in the region, four are still present but with greatly reduced and fragmented distributions, seven have broad distributions in the region little changed since Blandowski’s time, although severely fragmented, and the remaining four have probably expanded their distributions. The contributions of Blandowski and his assistant Gerard Krefft to our understanding of the nature and causes of these mammal declines are examined and discussed. Unfortunately, the surviving contemporary documentation of the Expedition and the associated specimens is inadequate to shed much light on the factors that triggered the initial mammal declines, but the results do not support recent suggestions that predation by the introduced house cat Felis catus was pivotal.