Attention Deficit Hyperactivity Disorder (ADHD) is one of the most prevalent conditions among school children. Executive function deficits representing difficulties in maintaining an appropriate problem set for the attainment of future goals are reported to be the major deficit in ADHD populations. There is a high rate of co-morbidity of learning disabilities and ADHD, with empirical evidence indicating an association with math and reading difficulties, but there is little research on the written expression of this population. There is a body of emergent research indicating that written expression is mediated by executive function. Written expression is a complex task that is affected by motivation, working memory, cognitive processes and long term memory, factors which are reported to be compromised in ADHD populations. This study evaluated the working memory and fluid reasoning in children with (combined and predominantly inattentive types) and without ADHD. Second, it explored the relationship between working memory and fluid reasoning on written expression in children with and without ADHD. Finally, the possible link between the executive functions of working memory and fluid reasoning, with written expression of children with and without ADHD was examined. The findings of this study indicate that children with the combined type of ADHD had lower written expression and working memory scores compared to children with the inattentive type of ADHD. The results of this study also indicated an association between disinhibition and working memory deficits on written expression performance. This research will serve to contribute to an understanding of the functional impact of ADHD on academic performance. Findings from this study could potentially help with interventions for deficits in written expression among school children.