Drawing upon nine years of qualitative data, including a collection of writing samples and yearly interviews, this study seeks to articulate a model of long-term writing development that can be adapted for a wide range of research and teaching purposes. The model is adapted from Bronfenbrenner and Morris's Bioecological model of human development and draws upon key works by writing transfer scholars, longitudinal researchers, and the work in lifespan development. The model identifies the critical interplay of ecologies of writing specifically through the intersection of Person characteristics (e.g., Identities, Dispositions, and Resources) with Key Events over Time, nested in particular writing Contexts. We specifically focus on the way that various Person characteristics (including sociocultural, sociolinguistic, and socioeconomic), drastically shape writing development over Time, particularly as they are mediated by the Salience of the specific Writing Event and a writer's metacognitive awareness. Through case studies, we trace two writers' long-term development across nine years, spanning their undergraduate degrees, internship and workplace contexts, and for one writer, experiences in medical school contexts. With a model that can be applied to a variety of research and teaching contexts to better understand learners' writing development, we argue that Person characteristics--mediated by Salience and Metacognition and working together with Key Events, Contexts, and Time--substantially shape long-term outcomes for writing and learning. Through this robust model, we offer methodological and pedagogical implications.