How heritage learners successfully maintain their heritage language (HL) and literacy is a significant topic of discussion in the field of bilingual education. This study examines what factors are most closely associated with literacy competence by inspecting Korean heritage learners' language and literacy practice patterns and their literacy performance. Using language background surveys and Korean writing samples gathered from 56 Korean-English bilingual students in grades 4-12 and university undergraduates, it was discovered that participants' levels of Korean literacy were positively associated with home Korean language use and focused Korean language practice, whereas they were negatively correlated with years of US stay, years of schooling in the USA, and home English language use. A regression analysis revealed that the quality and range of HL practice was the only significant predictor of HL literacy skills. In addition, a one-way ANOVA indicated no significant difference for HL writing scores across four age groups (i.e. elementary school, middle school, high school, and college), which implies that HL literacy does not necessarily develop or accumulate with cognitive maturity or length of schooling, a finding dissimilar to conventional native language and literacy development.