Lesbian, gay, bisexual, transgender, and queer (LGBTQ) individuals in most countries face strong stigma and often rely on affirmative mental health care to foster coping and resilience. We tested an LGBTQ-affirmative mental health training for psychologists and psychiatrists by comparing in-person versus online modalities and the added benefit of supervision. Participants were randomized to a two-day training either in-person (n = 58) or via live-stream online broadcast (n = 55). Outcomes were assessed at baseline and 5, 10, and 15 months posttraining. Optional monthly online supervision was offered (n = 47) from months 5 to 15. Given the substantial need for LGBTQ-affirmative expertise in high-stigma contexts, the training took place in Romania, a Central-Eastern European country with some of the highest LGBTQ stigma in Europe. Participants (M age = 35.1) were mostly cisgender female (88%) and heterosexual (85%). Trainees, regardless of whether in-person or online, reported significant decreases from baseline to 15-month follow-up in implicit and explicit bias and significant increases in LGBTQ-affirmative clinical skills, beliefs, and behaviors. LGBTQ-affirmative practice intentions and number of LGBTQ clients did not change. Participants who attended at least one supervision session demonstrated greater reductions in explicit bias and increases in LGBTQ-affirmative behaviors from baseline to 15-month follow-up than participants who did not attend supervision. LGBTQ-affirmative mental health training can efficiently and sustainably improve LGBTQ competence and reduce provider bias in high-stigma contexts. Future research can identify additional ways to encourage mental health providers' outreach to LGBTQ clients in need of affirmative care.