Background <p>Sexual and gender diverse adolescents and young adults (SGDAYA) experience mental health disparities, yet few empirical investigations into the long-term impact of affirmative treatments on their well-being exist.</p> Methods <p>This study explored the longitudinal effects of a brief affirmative cognitive-behavioral therapy (CBT) group intervention (AFFIRM) on the depression and anxiety of SGDAYA (<i>N</i> = 202), as well as how pre-treatment and mid-intervention change mechanisms contributed to their improved mental health. Participants’ age ranged from 14 to 29 years old at baseline (<i>M</i> = 22.12, <i>SD</i> = 4.60). Data were collected at four time points (pre-test, post-test, 6 months, 1 year) and analyzed using hierarchical linear models.</p> Results <p>Participants reported significant improvements in anxiety and depression from baseline to 1-year follow-up as well as increased engagement coping, social support, hope, and improved stress appraisal. SGDAYA, who appraised stress as a threat and had less ability to envision a hopeful future (hope pathway) at baseline, reported greater improvements in depression and anxiety; additionally, those who used more disengagement coping strategies prior to AFFIRM reported more reduction in depression. Participants with the most significant long-term improvement in depression reported (a) greater increases in their resources to deal with stress, (b) more uptake of engagement coping, and (c) improved hope pathway.</p> Conclusions <p>This study suggests that an affirmative cognitive-behavioural group intervention designed for SGDAYA can have a long-term impact on their depression and anxiety and highlights the important role of engagement coping, social support, hope and cognitive appraisals on youth mental health.</p> Trial registration <p>AFFIRM was retrospectively registered as a clinical trial on March 24th, 2020 (identifier: NCT04318769).</p>