Introduction: Cognitive impairment (COIM) is a major challenge for healthcare systems and is associated with an increased risk of adverse outcomes in older people visiting emergency departments (EDs). Owing to global aging, both cognitive screening and comprehensive geriatric assessment (CGA) application in ED settings are developing areas of geriatric emergency medicine. Meanwhile, the association between clinical outcomes of COIM; cognitive impairment, no dementia (CIND); and dementia in the ED could be better investigated. Our study aims to identify individuals with COIM from older patients in the ED via CGA and to describe the association of CIND and dementia with prognosis in ED visits. Methods: A prospective cross-sectional study was conducted in the ED of the Taipei Veterans General Hospital, a medical center located in Taipei, Taiwan, from August 2018 to November 2020. Patients aged ≥75 years with and without COIM were compared using data obtained from the CGAs conducted by trained nurses. Results: A total of 823 older patients were enrolled in the study and underwent CGA. Of these, 463 (56.3%) were diagnosed with COIM, of which 292 (35.5%) were diagnosed with dementia; and 171 (20.8%), CIND. Between the no-COIM and COIM groups, the COIM group had a higher rate of hospital admission (p = 0.002) and mortality at 3 months (p < 0.05). Among the no-COIM, CIND, and dementia groups, ED disposition (p = 0.001) and the rate of revisit/readmission (p < 0.05) showed significant differences. In particular, the dementia group had a significantly higher rate of revisit/readmission as compared to the CIND group among the three groups. Discussion/Conclusion: Older patients with COIM had a higher rate of hospital admission and mortality at the 3-month follow-up than older patients without COIM. Among the no-COIM, CIND, and dementia groups, patients with dementia had significantly increased risks of hospital admission and revisit/readmission. The early detection of COIM, and even dementia, could help ED physicians formulate strategies with geriatric specialists to improve mortality outcomes and revisit/readmission.