This qualitative study explores Korean high school students' exercising of agency in processing and producing L2 writing. Data were collected from off-line and online interviews, field notes, and other written materials over the course of two years and analyzed from a social view of agency (Ahearn, in: Jaspers, Östman, Verschueren (eds) Society and language use, John Benjamin Publishing Company, Amsterdam, 2010; van Lier in Sociocult Theory Teach Second Lang 163:186-193, 2008). The students' engagement in varied L2 writing projects and their writing artifacts consistently showed their enhanced awareness of linguistic and other semiotic resources which resulted in their frequent and continuous use of multiple languages and other placed resources. Meanwhile, they developed their strategies and reshaped their L2 writing practices considering the given context, placed resources, and their funds of knowledge. Findings from this study provide valuable insights into the open possibilities of EFL students' exercise and development of agency, which is an increasingly necessary feature of life-long learners in the post-pandemic era.