Slender mongooses are primarily carnivorous, though they are opportunistic omnivores. Insects make up the bulk of their diet, but lizards, rodents, snakes, birds, amphibians, and the occasional fruit are eaten when available. They can change their diet depending on habitant and the season. Slender mongooses are adept at climbing trees, where they often hunt nesting birds and search for eggs. They will also eat carrion.
Slender mongooses store and cache food in burrows for up to several days, especially when caring for young. These mongooses are diurnal foragers that forage primarily mid-morning to late afternoon.