Working memory deficits in children with ADHD are well established; however, insufficient evidence exists concerning the degree to which lower-level cognitive processes contribute to these deficits. The current study dissociates lower level information processing abilities (i.e., visual registration, orthographic conversion, and response output) in children with ADHD and typically developing children and examines the unique contribution of these processes to their phonological working memory performance. Thirty-four boys between 8 and 12 years of age (20 ADHD, 14 typically developing) were administered novel information processing and phonological working memory tasks. Between-group differences were examined and bootstrap mediation analysis was used to evaluate the mediating effect of information processing deficits on phonological working memory performance. Results revealed moderate to large magnitude deficits in visual registration and encoding, orthographic to phonological conversion, and phonological working memory in children with ADHD. Subsequent mediation analyses, however, revealed that visual registration/encoding alone mediated the diagnostic group status/phonological working memory relationship and accounted for approximately 32% of the variance in children's phonological working memory performance. Diagnostic and treatment implications for understanding the complex interplay among multiple cognitive deficits in children with ADHD are discussed.