Background: Teenage youth with co-morbid intellectual disability (ID) and autism spectrum disorder (ASD) could be at higher risk for behavioral and social difficulties than youth with ID only. Methods: Youth aged 13 years were diagnosed with ID (n = 37), ID/ASD (n = 26), or typical development (TD, n = 98). Mothers and teachers completed measures of youth behavior problems, mental health, social skills, friendships, and their own well-being. Results: ID/ASD youth were at heightened risk across almost all measures when compared with TD youth. On 14 of the 38 domains assessed, ID/ASD youth had significantly more difficulties than youth with ID only. Most notably, they scored higher on internalizing problems and lower on friendships and peer acceptance. Their mothers reported lower well-being for themselves. Conclusion: Compared to families of youth with ID only, youth with combined ID/ASD experienced greater psychological and social difficulties and their mothers experienced lower-well-being.