This study examines the developmental patterns of existential constructions in the interlanguage of Korean learners of English. Grounded in typological contrasts between topic-prominent and subject-prominent languages, the research investigates how Korean learners’ L1 influences their use of existential structures and how proficiency mediates this process. Ninety-six Korean university students were assigned to lower-, middle-, and upper-proficiency groups, and 20 English native speakers as a control group. Data were collected through a writing task and a discourse completion task. Quantitative analysis revealed that lower-proficiency learners predominantly relied on topic-comment and “have” type constructions, reflecting strong L1 transfer. In contrast, upper-proficiency learners demonstrated a higher frequency of target-like existential sentences, approaching native speaker patterns. Chi-square analyses confirmed significant associations between proficiency level and sentence type selection. The findings underscore the gradual acquisition of existential constructions and the enduring impact of L1 information structure. Pedagogical implications highlight the need for explicit instruction addressing both form and discourse-pragmatic functions of existential constructions.