Objectives: Providing support to people with intellectual and developmental disabilities (IDD) can be a major source of stress for the family caregivers. It is therefore important to identify resilient and protective factors for caregivers. This study examined whether mindfulness facets predict over time fewer symptoms of anxiety and depression and better quality of life for the family caregivers of people with IDD. Methods: A sample of 143 Spanish relatives of people with IDD (122 women and 21 men), aged between 18 and 78 (M = 46.87, SD = 12.16), participated in a two-time 6-month longitudinal study. Participants completed measures of anxiety, depression, quality of life, and mindfulness facets at the beginning of the study. Six months later, they again completed measures of psychological symptoms and quality of life. Results: The findings indicated that acting with awareness and non-reacting were the most beneficial mindfulness facets for caregivers. Acting with awareness predicted a reduction of anxiety (β = − 0.16, p < 0.01) and depression (β = − 0.17, p < 0.05) and an increase in quality of life (β = 0.19, p < 0.05) at the follow-up. Non-reacting predicted a reduction of anxiety (β = − 0.16, p < 0.05) and depression (β = − 0.13, p < 0.05) over time. Conclusions: The results of the study show the beneficial role of mindfulness facets and suggest the importance of studying the effect of mindfulness-based interventions for the relatives of people with IDD.