An annotated checklist of the terrestrial mammals of Los Angeles County was compiled based on over 3,000 museum records. A total of 84 species of terrestrial mammals have been recorded from Los Angeles County, of which 42 species from three orders are included in Part I: Didelphimorphia, Lagomorpha, and Rodentia. Each species annotation includes a brief synonymy and description of its recorded distribution in the county. Although approximately 38% of Los Angeles County is developed or urban, it supports a diverse terrestrial mammal fauna that is mostly found in large natural and open space areas, such as the San Gabriel Mountains, Santa Monica Mountains, Santa Susanna Mountains, Griffith Park, Verdugo Mountains, Palos Verde Hills, Puente Hills, San Rafael Hills, Soledad Canyon, and Mojave Desert.