Background: While physicians tend to emphasize on physical medical problems, behavioral and cognitive disorders of geriatric patients are usually missed, especially in emergency settings. The aim of the study was to determine the prevalence of delirium, dementia, and depression (3D) among older patients (>= 65 years old) in the Emergency Department (ED) and to evaluate the effect of geriatric 3D on the 6-month and 5-year mortality. Materials and methods: This was a prospective, observational cohort study, including 415 patients from eligible 512 consecutive older patients, who are 65 years of age or older, presenting to the ED of a tertiary care university hospital. Geriatric delirium, dementia, and depression were prospectively evaluated using Confusion Assessment Method, Quick Confusion Scale, and Geriatric Depression Scale-15, respectively. Premorbid functional status was determined by Barthel Index. The Charlson Comorbidity Index was used to measure the comorbid burden. After enrollment, patients were screened for 6-month and 5-year survival rates via the Government Death Reporting System records. The Kaplan-Meier method and Cox proportional hazards analysis was used for survival analysis. Results: Among the study population, the prevalence of geriatric 3D was found as 10.6% (n = 44/415) for delirium, 45.6% (n = 160/351) for dementia, and 35.1% (n = 123/350) for depression. Delirium, dementia, and depression all had higher mortality rates among older ED patients covering the 5-year period. However, only delirium was predictive of both 6-month and 5-year mortality rates. Conclusion: Aside from the medical and surgical issues of geriatric patients, the prevalences of dementia and depression are much higher than expected in the emergency department. Delirium was a predictor for 6-month and 5-year mortality. We suggest that EDs should have screening tools for geriatric 3D mental health disorders to increase the quality of life for the geriatric population.