In this study we examine processes of motivational change for middle school language learners (N = 174) within the classroom ecology of genre-based writing. Through longitudinal cluster analysis, we investigate change in language learners’ writing-specific motivational profiles, supported by a time-series analysis of reflective journals and interviews with these participants. We first identified several distinct initial motivational profiles for the middle school L2 learners who were part of the study and then traced key patterns of change for each. Our data indicate that over the course of genre-based L2 writing instruction, the students with respective profiles were able to develop a stronger capacity for writing self-regulation and to sustain and consolidate their writing self-efficacy. One unique finding was that the final profile of many learners was characterized by elevated levels of writing anxiety. However, because this was combined with moderate-to-strong levels of writing self-regulation and writing self-efficacy it suggests that anxiety can co-exist in constructive configurations, such as alongside adequate levels of self-regulation and self-efficacy that can offset this. We discuss the contribution of our findings in relation to understanding how L2 learners experience motivational changes in instructional settings of L2 writing.