The aims of the current project were to: (a) describe lexical diversity, morphologically complex word use (MCW), and writing quality ratings of 5th-grade students with diverse language backgrounds and (b) to examine the relationship between analytical components and writing quality ratings with performance on measures of language and literacy achievement. This study examined 1,825 written responses of 5th-grade English Learners with limited English proficiency (LEP) compared to English proficient speaking peers with and without language-learning disorders (LLD). Structural equation modeling was conducted to examine the relationship between analytical and holistic writing measures with achievement. Findings demonstrated differences in analytical components and writing quality between groups, evidencing lower performance for students with LEP and students with LLD. Students' lexical diversity and MCW use in written responses were strongly related to overall quality ratings and significantly related to performance on standardized assessments. Confirmatory structural equation modeling results suggested that the latent variable of analytical language had a strong relationship with writing quality ratings and a small but significant relationship with language and literacy achievement. Results suggest that the relative contribution of writing skills to achievement was not significantly different between students proficient in English and ELs with LEP.