ADHD is associated with processing speed (PS) deficits and mathematic difficulties. However, the relationships between PS and mathematics in children with ADHD were understudied. The current study investigated the contributions of PS to math fluency (MF) and tested the mediating role of working memory (WM) in the relationships between PS and MF in children with ADHD. Seventy-eight third to fifth graders (ADHD children, n = 52; Typically developing children, n = 26) were tested on their PS (perceptual, graphomotor-cognitive, and phonological), MF, and WM using standardized measures. Hierarchical regression analyses showed that only graphomotor-cognitive PS significantly predicted MF in children with ADHD. Besides, mediation analyses revealed that graphomotor-cognitive PS had both direct and indirect effects via WM on MF. Although such results suggested that slow PS contributed to impaired WM, the current study is unable to determine the directionality of effects due to the nature of research design.