There is a need to extend and test the feasibility and acceptability of mental health outcome measures in the older population (i.e., aged 65–100). We present data on the CORE-OM (Clinical Outcomes in Routine Evaluation-Outcome Measure) on a sample of 118 people aged 65–97 presenting for mental health treatment and 214 people aged 65–94 drawn from a non-clinical population. Results show the CORE-OM to be a reliable measure in both samples when the overall mean item is used but the reliability is not as high for the specific domains as psychometrically stable structures. The CORE-OM showed large overall differences between the non-clinical and clinical samples indicating that it is equally as sensitive to these differing populations across this older age band as with working-age adults. However, the norms for the clinical sample were consistently lower than the equivalent clinical norms for a working-age sample. These findings suggest that the collection and compilation of age-specific norms is crucial in ensuring that appropriately referenced norms are used rather than assuming that norms are generalizable across the whole adult life-span.