Social attention is a critical skill for learning and development. Social attention difficulties are present in both non-syndromic autism spectrum disorder (nsASD) and fragile X syndrome (FXS), and our understanding of these difficulties is complicated by heterogeneity in both disorders, including co-occurring diagnoses like intellectual disability and social anxiety. Existing research largely utilizes a single index of social attention and rarely includes children with intellectual impairment or uses a cross-syndrome approach. This study investigated whether multi-trait social attention profiles including naturalistic initial eye contact, facial attention, and social scene attention differ in preschool children with nsASD and FXS matched on developmental ability (DQ) and contrasted to neurotypical (NT) controls. The relationship between DQ, ASD severity, and social anxiety and social attention profiles was also examined. Initial eye contact related to social scene attention, implicating that naturalistic social attention is consistent with responses during experimental conditions. Reduced eye contact and lower social scene attention characterized nsASD and FXS. Children with nsASD displayed less facial attention than FXS and NT children, who did not differ. Lower DQ and elevated ASD severity associated with decreased eye contact in nsASD and FXS, and lower DQ was associated with lower social scene attention in FXS. Sex, social anxiety, and age were not associated with social attention. These findings suggest social attention profiles of children with nsASD are highly similar to, yet distinct from, children with FXS. Children with nsASD may present with a global social attention deficit whereas FXS profiles may reflect context-dependent social avoidance.