Post-traumatic stress disorder (PTSD) is associated with an increased risk of dementia in male veterans, but little is known in females and civilians. PTSD and comorbidities were abstracted from medical records from 1/1/1996 to 12/31/2001. Dementia incidence from 1/1/2002 to 12/31/2014 in 499,844 health care members aged 60+ years over an average of 8.2 years. Cox proportional hazard models were adjusted for age, demographics, and comorbidities. PTSD was associated with increased risk of dementia over an average of 8 years of follow-up (females: hazard ratio [HR] = 1.59, 95% confidence interval [CI] = 1.30-1.95; males: HR = 1.96, 95% CI = 1.51-2.55). There was a two-fold risk of dementia in those with both PTSD and depression (females: HR = 2.08; 95% CI = 1.66-2.59; males: HR = 2.06; 95% CI = 1.47-2.91) versus those without. PTSD was a risk factor for dementia in both sexes, with a heightened risk in those with comorbid depression.