Neuropsychological assessments, especially for suspected dementia, often emphasise the comparison of current and pre-morbid intellectual functioning. However, when assessing bilinguals, estimation of pre-morbid intellectual functioning may vary depending on which language provides the medium of testing. For bilingual Welsh-English speakers, testing in English only may yield an inaccurate profile of pre-morbid ability and hence affect the accuracy of subsequent diagnosis. We report the development and standardisation of a Welsh-language lexical decision task, Prawf Gweld y Gair, and evaluation of its suitability for assessing pre-morbid intellectual functioning in clinical groups. Standardisation with 101 healthy over 50s showed that scores correlated significantly with scores on other measures of crystallised intelligence and, when age and gender were taken into account, significantly predicted scores on a measure of non-verbal fluid intelligence. In subsequent evaluation with healthy older controls (n = 25), people who have Alzheimer’s, vascular or mixed dementia (n = 26) and Parkinson’s disease (n = 25), as predicted, there were no significant correlations with age, fluid intelligence or cognitive status in the clinical groups. Gweld y Gair shows promise as a Welsh-language test of premorbid intellectual functioning and may be useful for clinicians assessing the cognitive abilities of older Welsh speakers.