This study examined the role of word-level reading proficiency and verbal working memory in grade 4 and 5 students’ (N = 42; 23 boys) performance on a curriculum-based measure of narrative writing. Two outcomes were measured: correct minus incorrect word sequences (CMIWS; accurate-production of spelling and grammar in-text), and composition quality. CMIWS scores were moderately correlated with the holistic quality score. Word reading proficiency predicted CMIWS above and beyond the variance accounted for by gender, grade, handwriting automaticity, and working memory. Word reading proficiency also predicted composition quality controlling for gender and handwriting automaticity. Working memory, as measured by an updating task, was not a significant unique predictor of CMIWS or composition quality. Grade (5 > 4) and gender differences (girls > boys) were also found for CMIWS scores. Although handwriting automaticity was correlated with CMIWS scores and writing quality, it was not a unique predictor of either measure. The results provide further evidence of the sensitivity of the CMIWS index. They also highlight the importance of considering reading proficiency and handwriting automaticity when assessing children’s writing abilities and planning instruction for children with writing difficulties.