Widespread declines of small- to medium-sized, semi-arboreal mammals in the drier regions of Northern Australia are of global concern. These declines have been variously attributed to either disruption of available resources or increased predation pressure. We aimed to clarify causes of mammal decline in Northern Australia using a comparative methods approach, examining historical changes in the distribution of two arboreal mammals, the common brushtail possum (Trichosurus vulpecula) and the savanna glider (Petaurus ariel), and model drivers of their current abundance. We used single-season occupancy models to describe changes in the geographic range of P. ariel and T. vulpecula based on multiple-source occurrence data, from before and after 1993. We conducted spotlighting surveys in 2016 across the mesic savannas of the Northern Territory to identify environmental correlates of the current abundance of each species. Our results show that, within northwestern Australia, the geographic range (area where the probability of occupancy was ≥ 5%) has declined by 72% for T. vulpecula and 35% for P. ariel, between the historical and contemporary periods (before and after 1993, respectively). The abundance of each species varied substantially across the study area, but high T. vulpecula abundance was associated with high shrub density. We propose that areas with high shrub density are providing refuge for T. vulpecula, due to an increase in protection from predation by feral cats (Felis catus). Regardless of the driver, conservation management within Northern Australia should concentrate efforts on maintaining or increasing shrub abundance in tropical savannas. Our findings should also be viewed as an indicator of early stages of P. ariel decline and prompt targeted monitoring efforts.