Learners' self-initiated language learning with mobile technology occurring outside the classroom is often contextualized, heterogeneous, and idiosyncratic. In this study, we propose a time-series clustering methodology for researching informal mobile language learners' learning and development of another language, with a view to unravelling the essential uniquenesses and commonalities in learners' developmental processes. Intensive longitudinal writing samples from nine English learners in China were collected and analysed with the methodology proposed, which first depicted individual-level developmental trajectories of writing complexity that were often idiographic or individual-specific, and then distilled salient developmental patterns that transcended the individual heterogeneity. These typically occurring patterns across individuals demonstrated a more predictable and interpretable manifestation of informal language learners' developmental processes. Methodological and pedagogical implications of adopting the time-series clustering methodology are discussed.