Social instability frequently arises in group-living species, but the potential costs have rarely been investigated in free-living cooperative breeders, especially across different timeframes. Using natural observations, body-mass measurements and life-history data from dwarf mongooses (Helogale parvula), we determined the short- and longer-term consequences of a change in one of the dominant breeding pair. We found that a new breeder led to alterations in both collective and individual behaviour (i.e., increases in communal scent-marking, engagement in intergroup interactions, sentinel activity and within-group grooming), as well as reduced body-mass gain, further demographic changes and decreased reproductive success (i.e., fewer pups surviving to adulthood). The effects were particularly apparent when it was the female breeder who changed; new female breeders were younger than more experienced counterparts. Our findings support the idea that stability and cooperation are strongly linked and provide potential reasons for previously documented health and fitness benefits of social stability.