The goal of this study was to describe how underlying vocabulary knowledge manifests into vocabulary usage, and in turn, how usage predicts writing quality among adult basic education (ABE) learners. ABE learners were administered tasks that measured vocabulary knowledge, in the forms of both vocabulary breadth and depth. Participants were also given a composition writing task, and these samples were evaluated for overall writing quality and vocabulary usage. A mediating model was constructed to describe the relationships among variables. This model indicates that vocabulary depth is predictive of writing quality through the mediating variable of vocabulary usage after controlling for the direct contribution of vocabulary breadth. We found no evidence that vocabulary breadth contributes to writing quality when controlling for vocabulary depth and vocabulary usage. The results of the study reveal important relationships among vocabulary knowledge and vocabulary usage in written work that warrants further investigation in developing learners.