The current study explores functioning in individuals with co-occurring Autism Spectrum Disorder and Down Syndrome (ASD+DS; n = 23), individuals with ASD and cognitive impairment (ASD+ID; n = 99) and individuals with idiopathic ID (n = 38). ANCOVA results revealed that individuals with ASD+DS showed strengths in behavioral functioning compared to individuals with ID and more similar behavioral functioning to those with ASD+ID ([eta][superscript 2] = 0.12), with the exception of disruptive behaviors. Cognitive functioning ([phi][subscript c] = 0.41) and ASD symptomatology ([eta][superscript 2] = 0.11) were more comparable for children with ASD+DS and ASD + ID than for individuals with ID. Individuals with ASD+DS had the lowest overall adaptive skills ([eta][superscript 2] = 0.11). Findings highlight similarities between ASD+DS and ASD+ID groups, emphasizing the importance of ASD identification within the DS population to provide access to specific interventions.