Objective: To estimate the prevalence of serious mental illness and dementia among Medicare beneficiaries in the Health and Retirement Study (HRS). Methods: This study utilizes HRS-linked Medicare claims data sets and inverse probability weighting to estimate overall and age-specific cumulative prevalence rates of dementia and serious mental illnesses among 18,740 Medicare beneficiaries. Two-way tabulations determine conditional probabilities of dementia diagnoses among beneficiaries diagnosed with specific mental illnesses, and binary logistic regressions determine conditional probabilities of dementia diagnoses among beneficiaries diagnosed with specific mental illnesses, controlling for covariates. Results: Weighted prevalence estimates for dementia, schizophrenia (SZP), bipolar disorder (BPD), and major depressive disorder (MDD) are similar to previous studies. Odds of dementia diagnosis are significantly greater for beneficiaries diagnosed with SZP, BPD, or MDD. Conclusions: Co-occurring mental disabilities require further investigation, as in the near future increasing numbers of mentally ill older adults will need appropriate and affordable community-based services and supports.