Being able to read and write in English as a second language (L2) in the globalized world has been increasingly considered important to gain greater personal, academic, and business achievement. This study investigates English (L2) reading and writing abilities in relation to first language (L1) reading and writing abilities, L2 vocabulary knowledge, and L2 educational backgrounds. The L1-L2 reading and writing relationships were examined in terms of the linguistic threshold and linguistic interdependence hypotheses. Data from 108 undergraduate students who learned English in South Korea were analyzed using a path analysis. Results indicated that L2 vocabulary knowledge was the most important predictor of both L2 reading and writing. L1 reading scores predicted L2 reading scores, which supports the linguistic interdependence hypothesis. A positive relationship between L1 and L2 writing was found for L2 learners with greater L2 vocabulary knowledge, supporting the linguistic threshold hypothesis. L2 educational backgrounds were also found important, such that time living in an L2-speaking country was related to better understanding L2 reading passages, while test familiarity was associated with better L2 writing performance. The findings provide a more comprehensive understanding of L2 reading and writing abilities in relation to their relevant skills and educational backgrounds.