Learner agency plays a key role in self-regulated learning. Yet, there is a paucity of research into its role in the distance learning context. Using reflective narratives written by a distance learner of English in China, this longitudinal case study aims to investigate the ways in which learner agency mediates the language learning in the distance mode. Findings from the study suggest that agency has a major impact on learners' self-efficacy, identity, motivation, and metacognition--four constructs which are instrumental in determining language learning success, in particular in the distance learning context. Analysis of the data indicates that positive changes in these four constructs, in turn, lead to further agentic engagement, hence forming a virtuous circle of mutual enhancement. Implications are discussed in relation to fostering distance learners' goal awareness, enhancing their self-efficacy, maintaining their motivation, and encouraging metacognitive efforts.