Charles Hoy, a collector for the US Museum of Natural History, spent three years in Australia (1919-1922) collecting at more than a dozen locations around Australia. He collected over one thousand specimens of mammals of more than 100 species. This paper collates the species collected by location and provides detailed quotes from his field notes and letters. Typically, his field notes and letters describe his collecting localities, detail what fauna he was able to collect, detail what additional fauna he had expected to collect, and speculate on the major land use changes impacting on the fauna. His letters and field notes provide valuable insights into the status of the Australian fauna and the perceived threats to its persistence at that time. He recorded a range of threats to mammals in temperate Australia, and singled out the desert fauna as being particularly vulnerable. In contrast, he recorded few threats in tropical Australia. Introduced foxes and cats, poisoning and trapping techniques to control rabbits, land clearing and hunting in localized areas, and regular burning of the forests were important factors in temperate Australia. He provides information of the timing of decline of mammals, distinguishing animals in the size category of the larger native rodents (the first to decline) and those the size of small wallabies (a subsequent decline). His fieldwork indicated an approximate synchrony in decline of medium-sized mammals at two distant locations (Eyre Peninsula in South Australia and Tamworth in New South Wales) in the period 1915-1918, which coincided with the eruptive wave of the fox invasion. In addition, he recorded the decline of specific species that he attributes to a combination of disease and predation from feral cats (the native cats Dasyurus spp.) or predominantly to disease (koala Phascolarctos cinereus).