The dataset evaluated the conversion between Mini-Mental State Examination (MMSE) and Montreal Cognitive Assessment (MoCA) scores, considering different cognitive profiles in a large population of healthy older adults and individuals with cognitive decline within the spectrum of Alzheimer’s (AD) and Parkinson’s (PD) diseases. Identifying reliable conversion norms could support the development of tailored cognitive assessments and interventions. To this end, we applied log-linear smoothing equipercentile equating (LSEE) to derive conversion tables from MMSE to MoCA and vice versa. The reliability of the conversion was evaluated using the Root Mean Square Error (RMSE) within a train-test validation approach. These results provide a valuable tool for cognitive screening, facilitating the interpretation of scores across different cognitive assessment scales and supporting precision approaches in cognitive evaluation and rehabilitation.