Background: Parents are uniquely placed to support their child's development. Interventions which are designed to be delivered by parents therefore hold considerable promise, particularly for children with neurodevelopmental conditions that are associated with particular developmental strengths and challenges. Aims: This study worked in partnership with families from the Down syndrome community to adapt an evidence-based early language intervention for children with Down syndrome. Methods and Procedures: Six families with a 3- to 5-year-old child with Down syndrome participated in this mixed-methods exploratory study. Guided by aspects of Community-Based Participatory Research and Design-Based Research, iterative cycles of design, implementation, analysis, and re-design were implemented to produce an adapted intervention programme. Data were collected using record forms, surveys, observations, and focus groups. Outcomes and Results: Findings showed many aspects of the original programme were acceptable and feasible for families, but important adaptations were identified, including enhancing repetition and consolidation, reducing time pressures, tailoring to individual needs, smaller steps for learning, supporting engagement, and increasing visual support. Adapting the programme in these ways enhanced adherence, enjoyment and the child's active engagement. Conclusions and Implications: This study is the first to report the process of adapting an existing language intervention for people with disabilities and highlights the value of working with families to identify the best ways to support their needs. Our approach shows promise for supporting language development in this population and serves as a foundation for future research that aims to develop novel interventions.