The identification of mental health disorders and dementia in individuals with intellectual and developmental disability is complex. This is due to possible overlapping clinical presentations and the difficulty in separating the symptoms of mental health disorders and dementia from the characteristics of intellectual and developmental disability. Furthermore, the intrinsic limitations in communication and challenges faced in elucidating accurate information in the presence of these conditions lend additional difficulty. However, differentiating between these conditions is essential for establishing accurate diagnoses and instituting appropriate treatment. Moreover, even if the patient presents with a completely unrelated complaint, being aware of the base diagnosis is vital. This is especially important so that appropriate intervention plans and community resources can be activated to support the individuals and their families. Having said that, the limited time and resources that primary care doctors have access to in their clinical practice may pose questions regarding how possible it is for them to make such a differentiation. This article aims to suggest a practical approach to differentiating between these conditions in the primary care setting.