Objectives: The Mental Health Index (MHI) is widely used as a measure of mental health status, but has not been evaluated in the geriatric oncology population. This study evaluated the MHI-17 in a geriatric oncology population, to establish validity and scoring rules. Materials and Methods: The Carolina Senior Registry (NCT01137825) was used to obtain data for 686 patients with cancer 65 and older who completed the MHI-17. The 17-item patient-reported measure produces one total score summing across four domains: anxiety, depression, positive affect, and sense of belonging. Cronbach's alpha (α), confirmatory factor analyses (CFA), item-response theory (IRT) analyses, and differential item functioning (DIF) analyses were used to evaluate internal consistency and validity. Results and Discussion: The revised MHI retained the 13 best-fitting items from the MHI-17 and resulted in a final model that included two subscales: anxiety (four items, RMSEA 0.11; CFI 0.99; TLI 0.98) and depression (9 items, RMSEA 0.10; CFI 0.96; TL 0.95). IRT analyses of the four anxiety items indicated good fit (RMSEA 0.08) and precise measurement of adults with poor mental health, and the nine depression items also fit well (RMSEA 0.05). No meaningful differences were found by sex, education, or treatment stage. Scores were developed to provide meaningful norms. The new MHI-13 is a shorter, more accurate way to assess mental health in older adults with cancer and most importantly allows clinicians to separately identify anxiety and/or depression - a clinically important distinction as treatment differs among these two types of mental health impairment.