Individuals with autism spectrum disorder (ASD) often meet criteria for at least one additional psychiatric disorder. The present study evaluated the utility of the Mini International Neuropsychiatric Interview (MINI) in assessing co-occurring psychiatric disorders in children, adolescents, and young adults with ASD. Ninety-one percent of children/adolescents and thirty-one percent of young adults were diagnosed with one or more co-occurring diagnoses using the MINI. MINI diagnostic rates were comparable to those found in the literature on children/adolescents with ASD; however, in young adults, MINI diagnostic rates were lower relative to rates found in the literature on young adults with ASD. Implications for treatment, transitioning to adulthood, and the need for instruments developed specifically to diagnose co-occurring disorders in ASD are discussed.