Behaviours of concern (e.g. aggression) are often present in residents of long-term care (LTC) facilities diagnosed with dementia and may impact quality of life. Prior uncontrolled research has shown that an intervention involving sharing resident life histories may be effective in reducing aggressive behaviours and improving quality of life, perhaps by increasing staff empathy. We used a randomised controlled design, involving a considerably larger sample than previous investigations. We also examined staff perceptions of LTC resident personhood in relation to aggressive behaviour. Seventy-three residents were randomised to either a life history intervention (N = 38) or a control condition (N = 35). Ninety-nine nurses and care aides answered questionnaires about their own attitudes and the residents' behaviours and quality of life at baseline, post-intervention and at follow-up. Results of mixed-effects modelling indicated significant differences between groups in personhood perception and resident quality of life. Personhood perception mediated the relationship between the intervention and improved quality of life. We identified significant negative correlations between resident cognitive impairment and staff perceptions of resident personhood. Qualitative findings suggested that staff primarily changed their verbal interactions with residents following the intervention, which may be particularly helpful for residents with the most severe dementia. Our results indicate that LTC residents benefit when life histories are constructed with their families and shared with nursing staff.