Positive psychologists have published hundreds of empirical studies correlating positive personality traits with improved outcomes in mental health, physical health, academic and career success, resilience, relationships, and personal happiness. But there remains a dearth of research on the emergence and development of positive personality traits. This grounded theory, qualitative research sought to discover whether positive personality traits can be developed in adult mentoring relationships. Sixteen participants responded in structured interviews about the benefits of their mentoring experiences, and in addition to performing coding analysis as described by Strauss and Corbin (1990), the researcher also compared the answers to Peterson and Seligman's taxonomy of positive traits (2004). Unprompted participant responses overwhelmingly asserted increase of positive traits, as well as five other benefit categories. Improved traits appeared across a wide range of mentee characteristics, and situations, including negative ones, as long as mentors communicated unconditional positive regard and possessed desirable competencies. Social considerations of this research include the possibility that, in combination with therapies to address negative aspects of a client situation, therapists using intentional positive trait development could support recovery, resilience, hope, wisdom, thriving, and all of the benefits positive psychology has correlated to the presence of positive personality traits. Future studies building on this research may include a longitudinal study to understand what situations and character types are most conducive for positive trait development, as well as questions regarding which traits appear in which mentoring situations.