Within a social view of literacy, this paper reports a two-year ethnographic case study of an adolescent multilingual writer, with respect to her transitions across in-school (i.e., Creative Writing class) and out-of-school writing contexts. This study was aimed to address two specific gaps in the fields of second language (L2) writing and literacy studies, by examining one of the most underrepresented groups (i.e., adolescent multilingual writers) and by exploring possible relationships between her voluntary, non-academic writing outside school and her academic writing practices. Findings reveal that a research participant greatly drew upon her voluntary, out-of-school writing for her Creative Writing class; and at the same time, her in-school writing activities and assignments were mentioned in out-of-school writing practices. This shows that her writing activities across in- and out-of-school contexts were influenced by each other, especially with respect to topics, genres, and languages of writing practices. These findings give us a more comprehensive understanding of the nature of adolescent L2 writers' composing practices and suggest a further examination of the relationships (both similarities and differences) between in- and out-of-school literacy learning and ways in which multilingual writers negotiate writing practices across varied writing contexts. (Contains 3 figures.)