This paper describes the development of a home-based occupational therapy intervention program for people with Down syndrome who experience early on-set dementia causing a decline in their performance skills and increasing care dependency on their informal caregivers. A six-step methodological process adapted from the Medical Research Council framework for developing and evaluating complex interventions was formulated to develop an evidence-based occupational therapy program for people with both Down syndrome and dementia and their informal caregivers. The first two steps gathered evidence through systematic reviews of the literature and determined the scope of current occupational therapy practice. The gathered evidence was synthesised in step three to develop a client-centred occupational therapy intervention program for persons with both Down syndrome and dementia and their informal caregivers. In steps four and five, opinions were sought from occupational therapists working in this area of practice on the content of the developed program and its feasibility within the Australian disability services context. The final testing step can be conducted in the future using a single-case experimental design study. It is important to use rigorous frameworks and gather comprehensive evidence using multiple methods to develop interventions for small heterogeneous populations. The developed occupational therapy program for persons with both Down syndrome and dementia and their informal caregivers appears feasible to be implemented within the Australian disability services; however, funding limitations imposes barriers for its implementation in clinical practice.