This study adopts Oxford's (2017) definition of language learning strategies (LLS) framed by self-regulation theory and explores the longitudinal development in the use of three second language (L2) writing strategies (global planning, local planning, L1-to-L2 translation) by Japanese university students as it interacts with cognitive, affective, and environmental variables. The study is innovative in its use of a comprehensive, theoretically supported definition of LLSs, a long observation period, and its use of an explanatory sequential mixed methods design. Mixed effects modeling, which provides the main statistical analysis, enabled us to simultaneously investigate the participants' strategy use at both group and individual level while relating these findings to the participants' emic data from a self-regulation perspective. The results reveal that (a) developmental trajectories in LLS use were differentially affected by other cognitive and environmental factors at both group and individual level, (b) the self-regulation perspective was useful in explaining the systematicity and individuality observed in the developing use of the three strategies, and (c) students' motivation related to study abroad experiences may be a key factor in understanding such trajectories.