Objectives: There is substantial evidence that cognitive behavioral therapy (CBT) and mindfulness-based cognitive therapy (MBCT) improve symptoms and functioning in adults with mood and psychotic disorders. There has been little work directly comparing these treatments among adolescents with early-onset mood or psychosis symptoms. Method: We conducted a randomized controlled trial comparing remotely administered group CBT to group MBCT for adolescents (ages 13–17) with a mood disorder or attenuated psychosis symptoms. Adolescents attended nine sessions over 2 months; their parents attended parallel groups focused on the same skill practices. Participants were assessed for psychiatric symptoms and functioning at posttreatment and 3 months posttreatment. Results: Sixty-six youth (Mage = 15.1 years, SD = 1.4; 44 females [66.7%]) initiated the trial (32 in CBT and 34 in MBCT), with 54 retained at posttreatment and 53 at the 3-month follow-up. The treatments were associated with comparable improvements in adolescents’ mood, anxiety, attenuated psychosis symptoms, and psychosocial functioning over 5 months. CBT was associated with greater improvements than MBCT in emotion regulation and well-being during the posttreatment period. MBCT (compared to CBT) was associated with greater improvements in social functioning among adolescents with greater childhood adversity. Both treatments had comparable rates of retention, but youth and parents reported more satisfaction with CBT than MBCT. Conclusions: The beneficial effect of both treatments in a group telehealth format is encouraging. Due to our limited sample, future research should investigate whether adolescents’ history of adversity and treatment preferences replicate as treatment moderators for youth with mood or psychosis symptoms.