Background: During the COVID-19 pandemic we implemented a partially online music intervention to examine the feasibility, experiences and short-term outcomes on the challenging behaviour and well-being of adults with intellectual disabilities. Method: This mixed-methods study included 10 participants with mild or moderate intellectual disabilities who received 16 one-hour individual music sessions in 10 weeks, either face-to-face or online. Data on feasibility and experiences from participants and music workers were collected and analysed using inductive thematic analysis. Challenging behaviour and well-being were measured before and after intervention. Results: Overall experiences were positive and concerned appreciation, positive feelings, musical abilities, attention span, relatedness and personalisation. Online experiences varied, but most participants preferred face-to-face over online sessions. After the intervention, challenging behaviour scores were better than before. Conclusions: Engaging in a partially online music intervention is feasible for people with intellectual disabilities and seems to improve challenging behaviour. Experiences are discussed and recommendations for future online sessions are provided.