Background: Culturally competent and equitable mental healthcare for LGBTQ+ people is critical for addressing mental health inequities for this population. Tools to assess therapists’ practice with LGBTQ+ clients are needed for research and clinical efforts related to mental healthcare equity goals. Methods: We conducted a preliminary assessment of the reliability and validity of a novel 28-item self-report measure assessing therapist practice with LGBTQ+ clients. We examined the construct validity using factor analyses, the convergent and criterion validity using intercorrelations with LGBTQ-affirming knowledge, self-efficacy, and attitudes, and the internal consistency using Cronbach alpha. Results: Our overall total LGBTQ+ practice measure demonstrated excellent internal consistency (Cronbach’s alpha = 0.91) and was strongly associated with LGBTQ+ knowledge (rho = 0.377), self-efficacy (rho = 0.633), and LGBTQ+ attitudes (rho = 0.305). We also identified two subscales: “Commitment to Continued Learning” and “Affirmative Practices”, which demonstrated similarly strong internal consistency and tests of validity. Conclusions: Our novel measure of overall LGBTQ+ practice, including two subscales, demonstrated strong reliability and validity. These findings have important implications for practice and research in mental healthcare for LGBTQ+ clients. Future research exploring these measures in relationship to mental healthcare outcomes is recommended.