Background There is no valid instrument for multidomain cognitive assessment to aid the detection of mild cognitive impairment (MCI) and mild dementia in Hong Kong. This study aimed to validate the Cantonese Addenbrooke’s Cognitive Examination Revised (C-ACER) in the identification of MCI and dementia. Methods 147 participants (Dementia, n = 54; MCI, n = 50; controls, n = 43) aged 60 or above were assessed by a psychiatrist using C-ACER. The C-ACER scores were validated against the expert diagnosis according to DSM-IV criteria for dementia and Petersen criteria for MCI. Statistical analysis was performed using the receiver operating characteristic method and regression analyses. Results The optimal cut-off score for the C-ACER to differentiate MCI from normal controls was 79/80, giving the sensitivity of 0.74, specificity of 0.84 and area under curve (AUC) of 0.84. At the optimal cut-off of 73/74, C-ACER had satisfactory sensitivity (0.93), specificity (0.95) and AUC (0.98) to identify dementia from controls. Performance of C-ACER, as reflected by AUC, was not affected after adjustment of the effect of education level. Total C-ACER scores were significantly correlated with scores of global deterioration scale (Spearman’s rho = −0.73, P < 0.01). Conclusion C-ACER is a sensitive and specific bedside test to assess a broad spectrum of cognitive abilities, and to detect MCI and dementia of different severity. It can be used and interpreted with ease, without the need to adjust for education level in persons aged 60 or above.