Narrative identity is the internalized, evolving story of the self that each person crafts to provide his or her life with a sense of purpose and unity. A proliferation of empirical research studies focused on narrative identity have explored its relationship with psychological well-being. The present study is the first prospective, multiwave longitudinal investigation to examine short-term personality change via an emphasis on narrative identity as it relates to mental health. Forty-seven adults wrote rich personal narratives prior to beginning psychotherapy and after every session over 12 assessment points while concurrently completing a measure of mental health. Narratives were coded for the themes of agency and coherence, which capture the dual aims of narrative identity: purpose and unity. By applying in-depth thematic coding to the stories of participants, the present study produced 47 case studies of intraindividual personality development and mental health. By employing multilevel modeling with the entire set of nearly 600 narratives, the present study also identified robust trends of individual differences in narrative changes as they related to improvements in mental health. Results indicated that, across participants, the theme of agency, but not coherence, increased over the course of time. In addition, increases in agency were related to improvements in participants' mental health. Finally, lagged growth curve models revealed that changes in the theme of agency occurred prior to the associated improvements in mental health. This finding remained consistent across a variety of individual-difference variables including demographics, personality traits, and ego development.