Background Dementia prevalence is increasing, and risk reduction is a priority. Online interventions that are useable and acceptable to people seeking to reduce their dementia risk are needed. We aimed to assess and explore end‐user perceptions of a large‐scale multidomain online intervention targeting modifiable lifestyle risk factors for dementia. Method Mixed methods evaluation involving Maintain Your Brain (MYB) randomised controlled trial participants 55‐77 years of age at baseline who engaged in assessments, brain training, nutrition and physical activity, and mental health modules across three years. Of 5,260 participants invited, 2,386 completed online surveys at trial conclusion of whom 1,589 offered free‐text comments. Of survey respondents, 248 agreed to be contacted for interviews, 70 were invited and 40 were interviewed. Survey respondents rated ease of use and overall experience with MYB, endorsed whether they would recommend MYB, and provided comments. Interview completers further rated satisfaction with support, expressed online or face‐to‐face setting preference, answered open‐ended questions about participation, activities and modules, and provided further comments. Data were summarised using descriptive statistics and explored using multiple qualitative methods. Result Respondents found computer‐based activities easy to use (92.2%), were satisfied with their overall experience (87.4%) and would recommend MYB (82.7%). Survey free‐text and interview comments addressed topics including computer literacy, health literacy, resource access, motivations, expectations, social relationships, and significant events. Perspectives on activities and information varied. Many enjoyed and/or appreciated the challenge of completing activities and assessments and the potential for health benefits from these activities. Others were disappointed by their own performance in activities and assessments, found testing and reporting repetitious, stressful and/or time‐consuming, and found educational information unhelpful. Many respondents would have preferred to access applications on phones or tablet devices and suggested that interventions should be available across platforms. Respondents wanted researchers to know that flexible and adaptive individually tailored goals and activities and timely feedback are important. Conclusion The MYB online multidomain dementia risk reduction program was useable and acceptable. Similar programs should be accessible, well‐supported, cross‐IT platforms, and incorporate adaptive, individually tailored activities and feedback that is responsive to the varied and changing needs of older adults.