This present study explores the functions of L1 as a language resource in an L2 collaborative writing group. Specifically, it aims to determine how the interlocutors’ first language produces power and identity in the production of knowledge during the pre-writing stage. Participated by selected junior high school students in a premier public high school in the Division of Cavite in the Philippines, the researcher recorded and transcribed the small group’s conversations into episodes to examine the linguistic choice of the participants that elicit power and identity building using their first language in an L2 learning space. Guided and influenced by Vygotsky’s sociocultural theory, which highlights language as a cultural artifact that upholds dialectical relation between the learners and the social world, results revealed that the students’ L1 could serve as a language resource in sustaining their voice necessary to build power and identity. This is manifested by the learners’ use of code-switching and code-mixing in ESL classrooms to empower them as significant members of the learning groups. Interestingly, pure L1 also proved that it has functions to play in producing and sharing authentic knowledge necessary to learn L2 and to establish their learning identity and agency. Hence, this study proves that local and authentic knowledge reflected in a bilingual learning engagement produces cohesiveness in L2 writing.