This narrative review examines the long-term effects of music therapy on individuals with intellectual disabilities (ID), including those with comorbid conditions. A systematic search of Scopus and Web of Science identified 684 records, with empirical studies meeting strict longitudinal and methodological criteria retained. The review is structured around five key dimensions: (1) participant characteristics, including ID severity and co-occurring conditions; (2) research designs and methodologies; (3) types of music therapy interventions and their comparative long-term efficacy; (4) targeted cognitive, emotional, and social abilities; and (5) the overall long-term effectiveness of music therapy. Additionally, the review investigates contextual variables and processes of change, offering a nuanced perspective on the long-term impact of music therapy. Findings reveal significant gaps in the literature, particularly the scarcity of longitudinal and follow-up studies, thereby limiting conclusive insights into sustained benefits. This review underscores the pressing need for robust, long-term research to strengthen the evidence base and optimize the use of music therapy in clinical and educational settings. Such efforts are essential for enhancing cognitive functioning, emotional regulation, social interaction, and overall well-being among individuals with ID.