Background: Children with autism spectrum disorder (ASD) demonstrate highly variable writing skills. Few studies have examined if engagement during writing assessments may differ for children with ASD and if task engagement is related to their writing assessment performance. This study examined narrative writing and broad task engagement in children with ASD compared to peers with attention-deficit/hyperactivity disorder (ADHD) and typically developing (TD) peers. Method: Sixty children with ASD, 32 children with ADHD, and 29 TD children completed assessments of cognitive skills, symptom severity, and spontaneous narrative writing. Time spent engaged during writing was assessed during the spontaneous narrative writing task. Results: The ASD group performed lowest on text organization and quality scores as well as word production scores while also spending the least time engaged with the writing task. Time spent engaged was most strongly associated with narrative writing scores in the ASD group and explained unique variance in text organization and quality scores and word production scores after controlling for related age, cognitive skills, and symptom severity variables. The ADHD group showed similar associations between time spent engaged and word production scores, and time spent engaged explained unique variance in word production scores. Conclusions: Time spent engaged completing the writing task appeared lowest for the ASD group and may suggest writing task engagement to be a more prominent difficulty area for children with ASD compared to peers with ADHD and TD peers. Implications for better understanding and supporting the writing skills of children with ASD are discussed.