Motivated by the effort of diagnostic interventions for EFL learners in Korean educational contexts, this study aims to identify heterogeneous L2 reading skill profiles among students attending the same school. Subsequently, it investigates the impact of learners’ L2 learning backgrounds on heterogeneity of these reading skills. Using Latent Profile Analysis on 234 vocational high school students, we identified four distinct profiles: Profile 1, severely weak L2 readers with very low decoding and syntactic knowledge; Profile 2, moderately weak L2 readers; Profile 3, above-average L2 readers; and Profile 4, fairly well-developed L2 readers with good vocabulary breadth. Multinomial regression analysis revealed that profile membership was significantly predicted by positive past learning experiences, extra-curricular English reading, and motivational attitudes. These findings underscore the significant heterogeneity in L2 reading skills within the seemingly homogeneous EFL group and highlight the critical association with past and current educational experiences, emphasizing the importance of tailored interventions based on individual learning histories.