This mixed-method single case study aims to unveil the plausible effects of extensive reading (ER) on an EFL learner’s writing competence and to elucidate the viable factors stimulating his willingness to write (WTW) after reading. The writing competence indicated by complexity, accuracy, and fluency was measured by eight measures under the CAF triad (Skehan, 1989). Narrative inquiry was then employed to probe students’ perception of how ER propelled his writing growth and leveraged his WTW based on two semi-structured interviews, excerpts from frequent contact via Facebook Messenger, and 51 reading logs composed during the 9-week course. Results showed observable progress in his writing accuracy and fluency, which was reaffirmed by the interview data. Meanwhile, his WTW was found to be attributable to five palpable determinants, namely (1) habit of L1 reading, (2) intrinsic motivation, (3) perceived writing growth, (4) reading materials, and (5) subconscious writer identity.