The assessment of cognitive function in individuals of culturally and linguistically diverse background poses considerable challenges for the psychologist, particularly when English proficiency is limited. This study explored the effects of diverse cultural background and non-Western educational background on Wechsler Adult Intelligence Scale-Third edition (WAIS-III) and Wechsler Memory Scale-Third Edition (WMS-III) performances in moderate-severe traumatic brain injury within an outpatient rehabilitation setting. Participants were aged 16–65 years and met careful selection criteria. IQ, index and age-scaled subtest scores were compared across three groups: (a) English-speaking background (n = 130), (b) culturally and linguistically diverse background and education completed in English (n = 33), and (c) culturally and linguistically diverse background and non-English education (n = 33). Cultural backgrounds included people of Asian, European, Middle Eastern, African and Oceania origin. Results were that the English-educated culturally and linguistically diverse group performed lower than the English-speaking background group on some verbal WAIS-III measures; effect sizes were small-moderate. The non-English-educated culturally and linguistically diverse group performed lower than both groups on several WAIS-III and one WMS-III measure, with large effect sizes. Clinical implications included the need for caution in interpreting test scores to avoid diagnostic errors and the need for further development of valid assessment tools.