Group-based interventions have been argued to slow the cognitive decline of older people residing in care by building social identification and thereby increasing motivation and engagement. The present study explored the identity–cognition association further by investigating the impact of a group decision-making intervention on cognition. Thirty-six care home residents were assigned to one of three conditions: an Intervention in which they made decisions about lounge refurbishment as a group, a Comparison condition in which staff made these decisions, or a no-treatment Control. Cognitive function, social identification, home satisfaction, and lounge use were measured before and after the intervention. Participants in the Intervention condition showed significant increases on all measures, and greater improvement than participants in both Comparison and Control conditions. Consistent with social identity theorizing, these findings point to the role of group activity and social identification in promoting cognitive integrity and well-being among care residents.