This is a dataset on mortality costs and reproductive success from intergroup conflict in banded mongooses, collected from a wild population of banded mongooses on the Mweya Peninsula, Queen Elizabeth National Park, Uganda between 2000-2019. We observed naturally occurring, aggressive interactions between social groups and recorded data on individual mortality occurring during and as a result of fighting. We used long term observations of reproductive events to determine individual reproductive success, measured as the total number of offspring assigned to each individual over the lifetime, and the number of offspring born to each individual from extra-group mating. We recorded data on oestrus events in social groups to determine when focal and rival groups were in oestrus simultaneously, and whether they were involved in an intergroup interaction with each other. These data were collected to examine the fitness costs and benefits of intergroup conflict. Full details about this dataset can be found at https://doi.org/10.5285/a1f3d7e5-934f-4b12-ba7c-c372ce9df2c6