Parents of children with autism spectrum disorder (ASD) are at risk for high stress levels. Multiple child factors have been identified as predictors of stress in parents of children with ASD, but factors associated with stress in parents of children with ASD and comorbid intellectual disability (ASD-ID) are not well understood. This study examined the role of child's age, social impairment, executive functions and adaptive skills on parenting stress in mothers of children with ASD-ID. Mothers of 133 children with ASD-ID (aged 4-11 years) completed the Parenting Stress Index--Short Form, the Behaviour Rating Inventory of Executive Function, the Vineland Adaptive Behaviour Scales, and the Social Responsiveness Scale. Mothers' stress varied according to the child's educational stage and associated with poor metacognition. No associations were found between parenting stress, age and adaptive skills of children. Social impairment also predicted parenting stress above and beyond child factors. Theoretical implications are discussed.