Self-report scores on personality inventories predict important life outcomes, including health and longevity, marital outcomes, career success, and mental health problems, but the ways they predict mental health treatment have not been widely explored. Psychotherapy is sought for diverse problems, but about half of those who begin therapy drop out, and only about half who complete therapy experience lasting improvements. Several authors have argued that understanding how personality traits relate to treatment could lead to better targeted, more successful services. Here self-report scores on Big Five and Big Six personality dimensions are explored as predictors of therapy presentation, usage, and outcomes in a sample of community clinic clients (N = 306). Participants received evidence-based treatments in the context of individual-, couples-, or family-therapy sessions. One measure of initial functioning and three indicators of outcome were used. All personality trait scores except Openness associated with initial psychological functioning. Higher Conscientiousness scores predicted more sessions attended for family therapy but fewer for couples-therapy clients. Higher Honesty–Propriety and Extraversion scores predicted fewer sessions attended for family-therapy clients. Better termination outcome was predicted by higher Conscientiousness scores for family- and higher Extraversion scores for individual-therapy clients. Higher Honesty–Propriety and Neuroticism scores predicted more improvement in psychological functioning in terms of successive Outcome Questionnaire–45 administrations. Taken together, the results provide some support for the role of personality traits in predicting treatment usage and outcome and for the utility of a 6-factor model in this context.