It has been suggested that mentalizing abilities underlie the distinct profiles of autism characteristics observed in Cornelia de Lange (CdLS) and fragile X syndromes (FXS). However, traditional explicit mentalizing tasks have high language demands that may mask mentalizing abilities. We compared performance on traditional explicit tasks and an implicit anticipatory looking mentalizing task in children with CdLS (N=9), boys with FXS (N=9), autistic (N=22) and neurotypical (N=34) children. The groups showed divergent patterns of performance. Neurotypical children had higher explicit mentalizing scores than all other groups. However, neurotypical, FXS and CdLS groups showed better implicit mentalizing performance than autistic children. Both chronological age and receptive language ability correlated with explicit mentalizing scores in neurotypical children. In autistic children, there was an association between explicit mentalizing score and receptive language ability but not chronological age. Explicit mentalizing score was not associated with receptive language ability or chronological age in the CdLS and FXS groups. Neither chronological age nor receptive language ability correlated with implicit mentalizing task performance in any group. Despite having intellectual disability and autistic characteristics, findings demonstrate that children with CdLS and FXS are skilled at spontaneously mentalizing.