This research was carried out to explore the role played by the first language (L1) in young learners’ processing and storage of English vocabulary in English as a foreign language (EFL) context. A series of written and online vocabulary assessments were administered to two classes of 9-year-old Taiwanese children who were distinguished by a pedagogical difference. A computation of vocabulary gain between the two assessments showed that the control group, provided with L1 glosses, had learned and retained more new words, demonstrating that L1 contributed to foreign language (FL) vocabulary learning. The group also produced shorter reaction times than the experimental group who were pedagogically instructed to map English vocabulary directly onto pictures. Furthermore, the interviews, which took place immediately after the online assessments, showed that both groups had employed L1 translation to access English word meanings. The results suggest a possible role for the L1 in young learners’ processing and storage of FL vocabulary.