The first language (L1) of language learners is a rich source of language knowledge that bilingual teachers and learners can bring to classrooms to facilitate second-language (L2) acquisition. While previous research suggests that L1 can provide learners with cognitive, pedagogical, and psychological benefits, what functions that it performs and how it supports language learning has not been thoroughly explored. This study aims to investigate different functions of L1 use in a beginner English class for 17 adult migrants in Australia through analysing teacher-students and students-students classroom interactions. Data were collected from lesson recordings, classroom observations and two interviews with the English-Chinese bilingual teacher. This study provides a refined categorisation of L1 functions through speech data rather than teacher's self-reports. Results show that the L1 was frequently used by the teacher for pedagogical purposes, classroom environment control and social relationship building. In particular, the L1 was mainly used for eliciting answers, giving classroom instructions and explaining meanings, and it was employed by learners to ask questions, give responses when they lacked the necessary skills in the target language, and to offer peer assistance. This study has implications for bilingual language support and learning materials design for beginner adult learners.