Introduction <p>It is widely acknowledged that mental health services should be affirming and tailored to meet the needs of TNB people, though only limited research has addressed this empirically. This study examined mental health help-seeking experiences of TNB adults and tested whether, and how, TNB-affirming practice predicts therapy outcomes.</p> Methods <p>A sample of 232 TNB therapy clients (<i>M</i> age = 26.98 years) completed a questionnaire assessing treatment characteristics and experiences, ratings of affirmative therapy practice, aspects of the therapeutic relationship, and therapy outcomes. Data were collected in 2018 from a sample of TNB individuals living in Australia.</p> Results <p>Depression, stress, and anxiety were the most commonly cited reasons for seeking help, and a sizeable proportion also sought help for gender-related factors. TNB-affirmative practice was associated with greater client satisfaction and greater therapy effectiveness; these associations were mediated by aspects of the therapeutic alliance. The most frequently cited reason for treatment termination was a lack of practitioner understanding, and only 22% of those who had terminated treatment did so because their problems had been resolved.</p> Conclusions <p>Results suggest the extent to which mental health professionals are TNB-affirming has important implications for the outcomes of mental health support among TNB individuals. Results also point to unmet mental health needs that are at least partially related to a lack of affirmative mental health care.</p> Policy Implications <p>Findings support recommendations for affirming and inclusive mental health services and suggest that the therapeutic alliance may be one mechanism through which affirmative practice could enhance therapy outcomes. We discuss policy and professional practice implications and highlight the need for further research focused on developing and testing the efficacy of tailored therapy interventions for TNB adults.</p>