Consequences of theory of mind (ToM) development for daily social lives of children are uncertain. Five to 13-year-olds (N = 195) with typical development, autism, or deafness (both native and late signers) took ToM tests and their teachers reported on their social skills for peer interaction (e.g., leadership, group entry). Groups differed in both ToM understanding (with late-signing deaf children especially delayed even relative to autistic children) and peer social skills (with autistic children especially delayed even relative to deaf late signers). Crucially, for the typically developing hearing children and deaf children alike, ToM understanding independently predicted peer social skills over and above age, gender, language ability, and, for deaf children, status as native- or late-signer. These novel findings offer some of the best evidence to date of the relevance of ToM cognitions to real-world social behavior for both these groups. However, for those with autism spectrum disorders (ASD) the pattern was different. The apparent link of ToM to peer competence was not a direct one but instead was significantly mediated by language ability. Several possible explanations for this intriguing autism-specific result were also discussed.