Local food systems may facilitate agroecological practices that conserve nutrient, energy, and water resources. However, little is known about the potential for local food systems to scale beyond niche markets and meet a substantial fraction of total food demand. Here we estimate the upper potential for all existing US croplands to meet total US food demand through local food networks. Our spatially explicit approach simulates the years 1850 through 2000 and accounts for a wide range of diets, food waste, population distributions, cropland areas, and crop yields. Although we find that local food potential has declined over time, particularly in some coastal cities, our results also demonstrate an unexpectedly large current potential for meeting as much as 90% of the national food demand. This decline in potential is associated with demographic and agronomic trends, resulting in extreme pressures on agroecological systems that, if left unchecked, could severely undermine recent national policies focused on food localization. Nevertheless, these results provide a spatially explicit foundation for exploring the many dimensions of agroecosystem sustainability.