Since its original description nearly two hundred years ago, the late Pleistocene (Ensenadan—Lujanian SALMA) ground sloth genus Mylodon from South America has had a variety of taxonomic issues. The end result is that diagnostic characters for the genus and its only species, M. darwinii, include only skull, tibia, and astragalus features. While other postcranial remains have been found, they have been too fragmentary to provide any significant taxonomic characters. The rediscovery of a specimen of M. darwinii (previously listed as Glossotherium robustum) collected in the mid-1920s near Orient in Buenos Aires Province, Argentina and housed in the Field Museum of Natural History, provides an opportunity to expand the diagnosis for this taxon based on diagnostic skull and astragalus remains that are associated with an atlas and associated vertebrae, and most of the left forearm, carpus, and other elements of the manus. These bones all show features that distinguish M. darwinii from contemporaneous mylodontids (e.g., Catonyx, Glossotherium, Lestodon, Scelidotherium), as well as some simple differences in size. The suite of currently known morphological characters and reconstructed muscle attachments also indicate a general functional niche for M. darwinii. They suggest that its body weight was borne on the lateral side of the manus rather than the knuckles or unguals, and that Mylodon may have had a relatively high degree of manual dexterity or digital strength.