Background: This study describes the sociodemographic, clinical, and functional characteristics of a representative sample of service users in Italy. The supports provided by formal agencies, natural networks, and actual levels of quality of life (QOL) were assessed. Methods: The participants were 1,285 individuals with intellectual and developmental disability served by 23 different services. The influence of availability of support strategies, environmental factors, client characteristics, personal desires and goals, and support needs on current QOL status was investigated using multiple regression. Results: QOL outcomes were significantly explained by support needs, client characteristics, personal goals and desires, and marginally by the presence of support strategies and environmental factors. Further, only a minor effect was found from support activities for general QOL outcomes. Conclusions: The results confirmed that the personal outcomes could be predicted providing support activities aligned to the specific personal needs and goals, confirming the importance of a person-centred planning process.