Importance: Behavioral therapy and cognitive-behavioral therapy (CBT) programs targeting a single class of problems have not been widely implemented. The population of youths with common mental health problems is markedly undertreated. Objective: To determine the effectiveness of a new transdiagnostic CBT program (Mind My Mind [MMM]) compared with management as usual (MAU) in youths with emotional and behavioral problems below the threshold for referral to mental health care. Design, Setting, and Participants: This pragmatic, multisite, randomized clinical trial of MMM vs MAU was conducted from September 7, 2017, to August 28, 2019, including 8 weeks of postintervention follow-up, in 4 municipalities in Denmark. Consecutive help-seeking youths were randomized (1:1) to the MMM or the MAU group. Main inclusion criteria were age 6 to 16 years and anxiety, depressive symptoms, and/or behavioral disturbances as a primary problem. Data were analyzed from August 12 to October 25, 2019. Interventions: The MMM intervention consisted of 9 to 13 weekly, individually adapted sessions of manualized CBT delivered by local psychologists. The MAU group received 2 care coordination visits to enhance usual care. Main Outcomes and Measures: The primary outcome was change in mental health problems reported by parents at week 18, using the Strengths and Difficulties Questionnaire (SDQ) Impact scale (range, 0-10 points, with higher scores indicating greater severity of distress and impairment). Primary and secondary outcomes were assessed in the intention-To-Treat population at week 18. Maintenance effects were assessed at week 26. Results: A total of 396 youths (mean [SD] age, 10.3 [2.4] years; 206 [52.0%] boys) were randomized to MMM (n = 197) or MAU (n = 199), with primary outcome data available in 177 (89.8%) and 167 (83.9%), respectively, at 18 weeks. The SDQ Impact score decreased by 2.34 points with MMM and 1.23 with MAU, from initial scores of 4.12 and 4.21, respectively (between-group difference, 1.10 [95% CI, 0.75-1.45]; P <.001; Cohen d = 0.60). Number of responders (≥1-point reduction in SDQ Impact score) was greater with MMM than with MAU (144 of 197 [73.1%] vs 93 of 199 [46.7%]; number needed to treat, 4 [95% CI, 3-6]). Secondary outcomes indicated statistically significant benefits in parent-reported changes of anxiety, depressive symptoms, daily functioning, school attendance, and the principal problem. All benefits were maintained at week 26 except for school attendance. Conclusions and Relevance: In this randomized clinical trial, the scalable transdiagnostic cognitive-behavioral intervention MMM outperformed MAU in a community setting on multiple, clinically relevant domains in youth with emotional and behavioral problems. Trial Registration: ClinicalTrials.gov Identifier: NCT03535805.