The International Classification of Functioning, Disability and Health (ICF) is the World Health Organization's (WHO) standard for assessing individual functioning. Over the last decade, the ICF has been made more accessible for autism and ADHD through the development and validation of tailored shorter ICF versions for these diagnoses, ICF Core Sets. To further enhance their applicability in research and practice, these Core Sets have been operationalized and implemented on an online platform, the ICF CoreSets platform. Here, we describe the iterative development process of the CoreSets platform. This multiple-methods study examines user feedback on the operationalization of the Core Sets and the feasibility of the CoreSets platform as a functional assessment for autism and ADHD. We collected a total of 678 assessments from the CoreSets platform. Individuals diagnosed with autism and/or ADHD, their relatives, participants from the general population, and professionals completed and provided feedback on the usability of the CoreSets platform. Qualitative feedback via interviews and focus groups were also collected. Qualitative data were analysed via content analysis, while quantitative data were examined using univariate and descriptive techniques. Findings show that the ICF CoreSets platform is feasible and user-friendly, but areas for improvement were also indicated, leading to additional refinement of the operationalization and platform. The operationalization of the ICF Core Sets and their implementation in the CoreSets platform appears adequate for use in research and practice, particularly after revisions indicated by future users, and is now ready for psychometric standardization.