Background: Cognitive processes and mechanisms underlying the strong link between attention-deficit/hyperactivity disorder (ADHD) and social problems remain unclear. Limited knowledge also exists regarding a subgroup of youth with ADHD who do not have social problems. This study investigated the extent to which executive function (EF) mediated the association between ADHD and social problems and compared ADHD symptoms, comorbidities, and EF among ADHD youth with and without social problems and controls. Methods: The sample consisted of 279 youth with ADHD and 173 controls without ADHD (11-17 years of age) in Taiwan. Among youth with ADHD, two subgroups were further identified: 70 ADHD youth with social problems and 31 ADHD youth without social problems. EF was assessed with four subtests from the Cambridge Neuropsychological Test Automated Battery (Spatial span, Spatial working memory, Intra-dimensional/extra-dimensional shifts, Stockings of Cambridge). Social problems were assessed by mother and child reports. Results: Executive function measures in working memory and planning mediated the effect of ADHD on social problems, independent of age, gender, and IQ. The two ADHD groups with and without social problems did not differ in ADHD severity or comorbidities; however, ADHD youth without social problems performed better on certain measures of EF such as working memory, planning, and response inhibition. Conclusions: These findings suggest that deficits in some aspects of EF may account for social problems often experienced by youth with ADHD.