Introduction <p>Cognitive Behavioural Therapy (CBT) is the primary evidence-based treatment for major depressive disorder, with behavioural activation and cognitive restructuring skill acquisition proposed as key mechanisms.</p> Objective <p>The relationship between CBT skill acquisition and reductions in depressive symptoms and suicidal ideation in young people remains unclear.</p> Methods <p>Data were drawn from the Youth Depression Alleviation–Combined (YoDA-C) trial, a randomised controlled trial comparing CBT with fluoxetine or placebo in 15–25-year-olds diagnosed with major depressive disorder. Group-based trajectory modelling identified patterns of skill acquisition over 12 weeks. Demographic and clinical predictors of skill development were examined. Generalised linear mixed models (GLMMs) assessed associations between skill trajectories and changes in Montgomery–Åsberg Depression Rating Scale (MADRS) and Suicidal Ideation Questionnaire (SIQ) scores.</p> Results <p>Three trajectories emerged: Low-Stable (47.6%), Moderate-Improving (42.2%), and High-Improving (10.1%). The Low-Stable group showed minimal skill gains, while the Moderate- and High-Improving groups demonstrated moderate to substantial improvements. GLMMs revealed significant reductions in MADRS and SIQ scores across all groups; however, rates of symptom change did not differ significantly between trajectories.</p> Conclusion <p>CBT skill acquisition trajectories were not associated with differential symptom improvement, suggesting symptomatic recovery may occur independently of skill mastery. These findings highlight the potential importance of broader therapeutic factors, such as the therapeutic alliance, in moderating outcomes for young people with major depression.</p> Trial registration <p>Prospectively registered with the Australian New Zealand Clinical Trials Registry (ACTRN12612001281886).</p>