Background: For people with intellectual disabilities (ID), social networks play a key role in facilitating social inclusion, health, and quality of life. This study shows that a multi-informant approach to collecting social network data improves our understanding of the social worlds of people with Down Syndrome (DS). Method: A mixed methods egocentric network approach was employed to investigate 27 dyads comprised of people with DS and their family members as proxy reporters to examine variability in network characteristics across self- and proxy reports. Results: The self-reported total network size of people with DS was significantly smaller than the network size based on proxy reports. Significant differences were found between self- and proxy-reported networks with respect to most relationship groups. Proxy informants reported more "paid staff". Conclusion: Our study showed that multiple perspectives on the social networks of people with DS are advantageous for researchers, policy makers, and practitioners.