This study compared the reliability, validity, and measurement properties of the Supports Intensity Scale–Children’s Version (SIS-C) in children with autism and intellectual disability (n = 2,124) and children with intellectual disability only (n = 1,861). The results suggest that SIS-C is a valid and reliable tool in both populations. Furthermore, the results of multi-group confirmatory factor analyses suggest that measurement invariance can be established across the two groups but that latent differences are present. Specifically, children with autism and intellectual disability tend to have higher intensities of support needs in social activities across age cohorts (5- to 6-, 7- to 8-, 9- to 10-, 11- to 12-, 13- to 14, and 15- to 16-year-olds), and children with intellectual disability only tended to have stronger correlations among support need domains measured on the SIS-C. Implications for applying the SIS-C to assessment and support planning are described.