The present study aims at exploring Korean EFL learners’ lexical structure and its development by analyzing their response patterns in a word association test. Studies on word association have shown that native English children go through a series of lexical restructuring, including the syntagmatic-paradigmatic shift. In order to see if Korean EFL learners’ mental lexicon also goes through the same restructuring and if their mental lexicon is affected by their L1, the present study conducted a word association test and a word familiarity measurement. Fifty adult Korean learners of English at different proficiency levels and eleven native English speakers participated in the experiment. The results showed that Korean learners produced much less paradigmatic and syntagmatic responses compared with the native speakers. The proportion of L2 learners’ paradigmatic and syntagmatic responses increased, with the increase in proficiency and word familiarity. However, no clear sign of syntagmatic-paradigmatic shift was found. Paradigmatic responses were predominant from noun prompts, whereas syntagmatic responses were predominant from verb prompts. The learners’ lexicon was also found to be affected by their L1. The findings suggest that Korean learners’ mental lexicon is structured differently from that of English native speakers.