Students with Attention-Deficit/Hyperactivity Disorder (ADHD) are struggling writers. Yet no comprehensive model has been validated to explain their poor writing outcomes. This study aims to test whether an extended version of the Not-So-Simple View of Writing (NSVW) model can describe the effects of key abilities on writing performance in students with ADHD. The sample included students with and without ADHD who completed cognitive and academic measures in the Colorado Twin Project. A Multi-Group Structural Equation Model approach revealed that multiple broad cognitive abilities predicted student writing performance and basic writing skills predicted more advanced writing skills. Model fit was excellent both for a model with writing as a single latent variable (fully latent) and as interrelated manifest variables (partially latent). Furthermore, students with and without ADHD demonstrated comparable patterns of relationships among the variables in the model. Implications for the assessment of writing difficulties in students with ADHD are discussed.