Impostor phenomenon (IP), characterized by persistent self-doubt despite objective success, is prevalent during the early stages of counselor development, often emerging as individuals transition from training into professional practice. This study examined whether self-compassion mediates relationships between IP, resilience, and mental health among 281 counselors-in-training enrolled in departments accredited by the Council for the Accreditation of Counseling and Related Educational Programs. Mediation analyses showed that self-compassion fully mediated the effects of IP on resilience and depression and partially mediated the effect on anxiety. MANOVA results indicated that higher self-compassion corresponded with lower IP, anxiety, and depression, and greater resilience, with robust effects. Findings identify self-compassion as a developmental competency that supports balanced self-evaluation, emotional regulation, and sustainable counselor well-being. Integrating self-compassion training into counselor education and supervision may help developing counselors manage impostor-related distress, strengthen resilience, and promote ethical, sustainable professional practice.