<p>Interest in learner engagement in foreign language education has increased, yet research on its development across various timescales remains limited. Using a classroom-based longitudinal research design, this study sought to investigate the dynamism of classroom engagement over a 15-week semester and identify, from students’ perspective, the factors contributing to this dynamism. Questionnaires and written reflective journals were used to collect data from an intact first-year College English class at a key university in central China. Conducted over five time points, each time of data collection corresponded with the completion of one learning unit. A trend of continuous growth in all the three engagement dimensions (i.e., behavioral, cognitive, emotional) emerged from the quantitative data via questionnaire. Content analyses of students’ reflective journals indicated that the developmental trajectories were shaped through the co-influence of multiple learner-internal and learner-external factors. Students’ perceptions of improved language competence and their attitudes toward the English class / English learning in general comprised key learner-internal influences, while externally the increased familiarity with the teacher and among the students stood out as the most prominent factor. All these factors pertained to students’ basic psychological needs satisfaction in the College English class and kindled profound pedagogical implications.</p>