Mammalian community ecology asks how interacting biotic and abiotic processes at local and regional scales shape community assembly. Throughout the 100-year history of the American Society of Mammalogists (ASM), mammalogists laid the foundation for studying community assembly by documenting presence and abundance of mammals across different sites through time. During the past 50 years, mammalian community ecology has matured due to long-term studies, new perspectives, and advanced analytical approaches. From an initial focus on local communities and ecological time frames, community ecologists now integrate regional processes and evolutionary time frames. Examination of patterns of assembly expanded from taxonomic diversity derived from species identity and abundance to incorporate phylogenetic and functional diversity. Arid-land small mammal and island systems provide natural laboratories to examine community assembly. Worldwide, small-mammal communities in arid lands exhibit several general trends including low α-diversity, greater β-diversity, and structures comprising core and transient species. Despite broad similarities in ecological conditions, the relative dominance of proximate processes structuring communities varies between regions due to differences in factors such as small-mammal lineages and predictability of resource pulses. Island studies reveal how island size and isolation affect species composition, and how processes of community assembly and disassembly are often simplified, compared to the situation in mainland communities. Recent approaches document the contributions of speciation on, and dispersal to and between islands, to patterns of community assembly. Islands also serve as models of how humans affect mammal communities. Land use practices were altering ecosystems before the founding of ASM, and these changes have accelerated over the last 100 years. Habitat conversion and degradation combined with climate change are redefining mammal communities globally. We discuss the need to apply our understanding of community ecology in efforts to restore ecological function, structure, and composition to stem potentially large losses in mammalian diversity.