Williams (WS) and Down (DS) syndromes are characterised by roughly opposing ability profiles. Relative verbal strengths and visuospatial difficulties have been reported in those with WS, while expressive language difficulties have been observed in individuals with DS. Few investigations into the executive function (EF) skills of these groups have examined the effect of verbal/visuospatial task type on performance. Analogous verbal and visuospatial measures were administered to these populations within four EF domains: executive-loaded working memory (ELWM), inhibition, fluency and set-shifting. Performance in both groups was compared to that of typically developing (TD) children using regression techniques controlling for potentially influential cognitive/developmental factors. Individuals with WS showed the expected relative visuospatial difficulties, as indicated by poorer performance than TD individuals, on tests of ELWM and fluency. Individuals with DS displayed the expected relative verbal difficulty in the domain of set-shifting. In addition, each population showed pervasive deficits across modality in one domain; ELWM for individuals with DS, and inhibition for individuals with WS. Individuals with WS and DS showed EF difficulties in comparison to a TD group, but, their executive performance was affected by EF task type (verbal/visuospatial) and EF domain in different ways. While the findings indicated that EF in these populations is characterised by a range of specific strengths and weaknesses, it was also suggested that the relative verbal/visuospatial strengths associated with each population do not consistently manifest across EF domains. Lastly, syndrome specificity was indicated by the differences in groups’ performance patterns.