"Novel" urban ecosystems are the product of successive human restructuring imposing at least two selective filters over millennia. The first filter – the Late Quaternary extinctions ca. 10,000 to 50,000 years ago – represents an early anthropogenic trophic re-organization through megafaunal collapse. The second filter, the urban Anthropocene, is defined by habitat fragmentation and food subsidies. Both filters are expected to alter resource use in existing species. Here, I evaluate response to these dual anthropogenic filters by an assemblage of small to medium-sized mammalian carnivores spanning the last 55,000 years in southern California, USA. Excluding coyotes, the late-Pleistocene to Holocene Rancho La Brea (RLB) asphalt seeps in Los Angeles preserve five mesocarnivore species each represented by >=10 individuals: American badger (Taxidea taxus), bobcat (Lynx rufus), grey fox (Urocyon cinereoargenteus), striped skunk (Mephitis mephitis), and long-tailed weasel (Mustela frenata). While these five species persist in the area today, radiocarbon dates indicate that not all species inhabited RLB at the same time. Rather, almost all fossil specimens of bobcats, grey foxes, and striped skunks date from before the Last Glacial Maximum (LGM), with badgers largely a latest Pleistocene addition. No sampled mesocarnivore specimens date to the LGM itself, likely because of low sample size or taphonomy. In multiple ecologically relevant traits, RLB specimens differ significantly from historic (<100 years old) representatives of the same species from the same geographic area. Fossil mesocarnivores were generally larger - decreasing in body size toward the present by as much as 27% - and exhibited a greater degree of carnivory-adapted morphology, perhaps to fend off competition from now-extinct megafaunal predators. Postcranial morphology also reflects species-specific shifts in locomotor function, potentially a response to environmental change. Further, stable isotope analyses of carbon and nitrogen show within-species differences across time, although to what extent these differences reflect changes in diet rather than environment is uncertain. All species except the weasel demonstrate higher and more variable δ13C toward the modern-day than pre-LGM; striped skunk and grey fox also exhibit lower-on-average δ15N in the Holocene to Anthropocene than pre-LGM. Leveraging biogeochemical and ecomorphological techniques, this study examines complementary proxies for diet and resource use, illuminating the roots of today’s urban carnivores in the last Ice Age and highlighting potential responses to continuing anthropogenic change.