This study examined the progress of English as a foreign language (EFL) writers using the instructional framework of systemic functional linguistics (SFL) and the communities of practice (CoPs) model. The study participants comprised 11 first-year undergraduate students in Japan with intermediate-level English proficiency who were exposed to SFL in a single EFL classroom (CoP). The participants' genre understanding and meaning-making decisions when writing discussion essays were studied over two semester-long courses. To do so, their developmental changes were analyzed using pre- and post-instructional writing tasks. In particular, their ability to convey interpersonal meaning, such as through the use of modal verbs, was examined and compared between the pre- and post-tasks. To triangulate the findings, participants' genre awareness in relation to discussion essays was also examined using in-depth qualitative analysis of their self-reflective texts and peer assessments, based on a grounded theory approach. In the pre-writing task, it was apparent that the learners lacked understanding of the components of discussion essay writing. However, analysis of their post-instructional tasks revealed that most had begun to apply the language components required to convey interpersonal meaning in their discussion genre texts. These results suggest that the changes in learner's genre awareness and knowledge affected the lexicogrammatical features they used when writing discussion essays. Thus, this study concludes that applying the SFL framework to writing instruction enhanced EFL learners' awareness of textual meaning and their understanding of the function of discussion essay texts.