To examine the diagnostic accuracy of the Thai version of the Mini-Addenbrooke's Cognitive Examination (MACE-T) for detecting mild cognitive impairment and dementia. Participants consisted of 60 controls, 40 people with mild cognitive impairment (MCI) and 48 people with dementia. All participants completed the Thai version of the Addenbrooke's Cognitive Examination - III (ACE-T) and the MACE-T scores were obtained from items within the ACE-T. A cut-off score of 21/22 detected MCI with a sensitivity of 95% (95% confidence interval (CI): 86.1-100) and specificity of 85% (95% CI: 77.8-93.4). A cut-off score of 16/17 detected dementia with a sensitivity and specificity of 95.8% (95% CI: 88.2-100) and 85% (95% CI: 76.4-89.7), respectively. Using Bayes theorem, the MACE-T demonstrated high negative predictive values (> 94%) for diagnosing MCI and dementia, independent of their prevalence rates. Education was the only significant predictor of performance in the MACE-T. The MACE-T is a good short cognitive screening test with high diagnostic accuracy for screening for MCI and dementia.