Background Increasing evidence supports that mentalization deficits may have a role in the genesis of young age depression; however, few studies examined this issue in clinical populations. Methods Outpatients aged 14–21, suffering from various psychiatric disorders, were assessed using the Mentalization Questionnaire (MZQ), the Beck Depression Inventory (BDI), and the Childhood Trauma Questionnaire (CTQ), using data from age‐matched healthy students for comparison. The relationship between CTQ, MZQ, and BDI scores was examined at the cross‐sectional level, including mediation analyses, and longitudinally, in a subsample who underwent a psychotherapy intervention. Results Of 83 subjects, 33 (39.8%) had mentalization levels that were 1 standard deviation below those of comparison subjects. In the whole sample, the levels of mentalization were inversely associated with BDI (r = −.68, p < .001) and CTQ scores (r = −.30, p = .006). Moreover, MZQ scores mediated a large part of the effect of childhood trauma on depression (total effect: 10.6, 95% CI: 5.3, 15.9; direct effect: 6.5, 95% CI: 2.1, 10.8; indirect effect: 4.1, 95% CI: 1.5, 7.4). This effect was almost entirely explained by the Affect Regulation subscale. In patients re‐evaluated after four sessions (n = 37), the decrease in BDI scores correlated with the increase in MZQ scores (r = .40, p = .02). Conclusions In a juvenile clinical sample, deficits of mentalization abilities were associated with the severity of depression and explained part of the depressogenic effects of childhood trauma.