In neurodevelopmental disorders, unique profiles of executive control and attention appear to co-occur with poor motor coordination. However, less is known about how syndrome-specific cognitive profiles interact with motor control and impact behavioural outcomes in neurodevelopmental disorders such as Williams syndrome (WS) and Down syndrome (DS). Here we aimed to examine the extent to which specific components of executive function interact with gait control when performing cognitive dual-tasks (verbal fluency, digit span) in WS and DS. Spatiotemporal gait characteristics and intra-individual variability of gait were assessed in individuals with WS who were matched on spatial ability to individuals with DS, and chronologically age (CA) matched controls. During the concurrent verbal fluency task, the WS group had greater dual-task costs on spatiotemporal gait parameters and variability than CA controls. Conversely, individuals with DS had selective gait interference during the concurrent digit span task when compared to CA controls, but only under increased demands on cognitive control where there was greater variability in step timing in DS. The interrelationships between cognitive-motor interference and behavioural measures of executive functioning appeared to differentiate between WS and DS, and emphasise the importance of task modality in unpacking the executive control profile in these neurodevelopmental disorders. These findings support the notion that associated cerebellar-cortico abnormalities may produce quite distinct profiles of executive control across cognitive and motor domains that impact on behavioural outcomes in neurodevelopmental disorders.