This study investigates the application of Mixed Reality (MR) as a tool for cognitive training with mentally disabled adults. Using the action research methodology, the project was carried out in partnership with the APPACDM of Mirandela, Portugal. The work followed iterative stages of planning, action, observation, and reflection, focusing on the development and validation of a MR prototype designed for Cognitive Training. A conceptual model was proposed, integrating Extended Reality (XR) with machine learning to support personalized, immersive experiences tailored to individual cognitive profiles, supported by the feedback from professionals of the association. The prototype focused on digitizing existing paper-based exercises into interactive MR activities using the Meta Quest® 3 headset, chosen for its balance of affordability and performance while enabling MR through Passthrough. Exercises included association and arithmetic tasks using grab-and-drop mechanics, which also align with principles of occupational therapy. The system was designed with modularity in mind, allowing easy creation of new content and infinite question generation. Results suggest that MR can enhance engagement, autonomy, and the development of cognitive skills in adults with special needs, offering a scalable and inclusive educational approach adaptable to various institutional settings. This work demonstrates the viability of integrating XR technologies into cognitive training, contributing to more innovative and accessible learning environments for people with special needs.