This case study investigates how an ESL teacher's activity of self-directed journal writing can facilitate learning and function as a mediational tool for teacher professional development. The participant for this study is a native English speaker who taught an ESL freshman writing course in an American university. Since he had little time to consult with teaching experts, he decided to engage in his self-directed journal for reflecting on his teaching practices. A sociocultural perspective on human cognitive development is utilized as an analytical framework for tracing his development and capturing the transformative power of narrative within his developmental trajectory. Using grounded theory, his journal entries are analyzed to identify a series of stages that trace his professional development, supported by face-to-face interview data designed to co-construct an interpretation of his journal and to serve as a member check. The data analysis suggests that his journal became a powerful mediational space where he critically reflected on and systematically examined his teaching practices. By externalizing his thoughts and feelings, as he first recognizes his loss of self-regulation and then works toward regaining his sense of professional expertise, he begins to develop alternative ways of thinking about his current teaching practices and starts to embrace new modes of engagement in his L2 classroom.