The co-occurrence of lower full-scale intellectual abilities (FSIQ) and academic achievement deficits in children with ADHD is well established; however, the extent to which the relation reflects the influence of a general factor (g) deficiency or deficiencies in one or more specific intellectual abilities remains speculative and was the focus of the current investigation. Twenty-eight boys with ADHD-combined presentation and 26 neurotypical (NT) boys between 8 and 12 years of age were administered the WISC-IV and standardized measures of reading and math. FSIQ and achievement scores in both reading and math were significantly lower for the ADHD relative to the NT group; however, examination of WISC-IV index scores revealed that group level differences in FSIQ resulted from lower scores on two of the four specific intellectual ability indices—Working Memory (WMI) and Verbal Comprehension (VCI). Bias-corrected bootstrapped mediation analyses revealed that both WMI and VCI contributed uniquely to the ADHD-Academic Achievement relation. The contribution of WMI to ADHD-related academic underachievement reflected lower scores on the Letter-Number Sequencing (LNS) but not the Digit Span (DS) subtest. Both LNS and VCI explained ADHD-related differences in reading, whereas LNS alone explained ADHD-related differences in math. Collectively these findings suggest that strengthening deficient higher-level WM abilities, in conjunction with empirically based academic instruction, is needed to improve learning outcomes in children with ADHD.