Montessori-based interventions (MBIs) were developed to promote guided participation in meaningful activities by people with dementia patients. In this study, we assessed nursing home volunteers’ fidelity to an MBI, relying primarily on a qualitative descriptive design. We completed a deductive content analysis of eight volunteer interviews using the Conceptual Framework for Intervention Fidelity. We also calculated average volunteer and resident scores on the Visiting Quality Questionnaire (VQQ), which assesses volunteers’ and residents’ perceptions of visits. We found good evidence that volunteers attended scheduled visits, made use of pre-designed activities, and attended to training recommendations. Most reported enjoying the visits (VQQ $ \overline{x} $ = 6.12, standard deviation [SD] = 0.75) and receiving a positive response from residents (VQQ $ \overline{x} $ = 5.46, SD = 0.88). Nevertheless, use of pre-designed activities and response to the MBI was lower for volunteers working with residents who had late-stage dementia. Therefore, overall, fidelity depended on the cognitive status of the resident.