Children with autism spectrum disorder (ASD) demonstrate heterogeneous writing skills that are generally lower than their typically developing (TD) peers and similar to peers with attention difficulties like attention-deficit/hyperactivity disorder (ADHD). Recent evidence suggests children with ASD spend less time engaging in writing tasks compared to their peers, but previous studies have not examined how children engage with the task environments of writing tasks. This study used video observation data collected from 121 school-age children (60 children with ASD, 32 children with ADHD, and 29 TD children) to compare differences in visual attention and writing task behaviors and relationships between behaviors and age, cognitive skills, and ASD and ADHD symptom severity. Findings indicated that groups mostly spent time looking at and writing on the draft, though this was lowest in the ASD group. No differences were found between the ASD and ADHD groups after accounting for duration as a percentage of total task time used. Groups spent little time looking at their outlines and looking away from the task, with all groups spending relatively more time looking at the task picture. Time spent engaged with the draft showed a positive relationship with writing performance across groups, but a negative relationship between time spent looking at the task picture and writing performance only appeared for the ADHD group. The ASD and ADHD groups showed negative associations between draft engagement and ASD symptom severity but not ADHD symptom severity. Implications are discussed for understanding writing task engagement in research and instructional contexts. [This paper was published in "Autism Research."]