OBJECTIVE The authors investigated the associations of medical and lifestyle factors with the mental health of men in their 80s. METHODS This was a prospective study of a community-representative cohort of older men. Successful mental health aging was defined as reaching age 80 years with Mini-Mental State Examination score (MMSE) of 24 or more and Geriatric Depression Scale-15 items (GDS-15) score of 5 or less. RESULTS Of 601 men followed for 4.8 years, 76.0% enjoyed successful mental health aging. Successful mental health aging was inversely associated with age (hazard ratio [HR] = 0.87; 95% confidence interval [CI]: 0.81-0.94), non-English-speaking background (HR = 0.42; 95% CI: 0.21-0.85), and the consumption of full-cream milk (HR = 0.63; 95% CI: 0.45-0.89), and directly associated with high school or university education (HR = 1.92; 95% CI: 1.34-2.75) and vigorous (HR = 1.89; 95% CI: 1.17-3.05) and nonvigorous physical activity (HR = 1.50; 95% CI: 1.05-2.14). Marital status, smoking and alcohol use, weekly consumption of meat or fish, and a medical history of hypercholesterolemia, hypertension, diabetes, myocardial infarction, and stroke were not associated with mental health outcomes in men aged 80 years or over. CONCLUSIONS Three in four men who reach age 80 years undergo successful mental health aging. Factors associated with successful mental health aging include education and lifestyle behaviors such as physical activity. Lifestyle modification by means of increasing physical activity and reducing saturated fat intake may prove to be a safe, inexpensive, and readily available strategy to help maximize the successful mental health aging of the population.