Background Psychosocial interventions for adolescent mental health problems are effective, but evidence on their longer-term outcomes is scarce, especially in low-resource settings. We report on the 12-month sustained effectiveness and costs of scaling up a lay counselor–delivered, transdiagnostic problem-solving intervention for common adolescent mental health problems in low-income schools in New Delhi, India. Methods and findings Participants in the original trial were 250 school-going adolescents (mean [M] age = 15.61 years, standard deviation [SD] = 1.68), including 174 (69.6%) who identified as male. Participants were recruited from 6 government schools over a period of 4 months (August 20 to December 14, 2018) and were selected on the basis of elevated mental health symptoms and distress/functional impairment. A 2-arm, randomized controlled trial design was used to examine the effectiveness of a lay counselor–delivered, problem-solving intervention (4 to 5 sessions over 3 weeks) with supporting printed booklets (intervention arm) in comparison with problem solving delivered via printed booklets alone (control arm), at the original endpoints of 6 and 12 weeks. The protocol was modified, as per the recommendation of the Trial Steering Committee, to include a post hoc extension of the follow-up period to 12 months. Primary outcomes were adolescent-reported psychosocial problems (Youth Top Problems [YTP]) and mental health symptoms (Strengths and Difficulties Questionnaire [SDQ] Total Difficulties scale). Other self-reported outcomes included SDQ subscales, perceived stress, well-being, and remission. The sustained effects of the intervention were estimated at the 12-month endpoint and over 12 months (the latter assumed a constant effect across 3 follow-up points) using a linear mixed model for repeated measures and involving complete case analysis. Sensitivity analyses examined the effect of missing data using multiple imputations. Costs were estimated for delivering the intervention during the trial and from modeling a scale-up scenario, using a retrospective ingredients approach. Out of the 250 original trial participants, 176 (70.4%) adolescents participated in the 12-month follow-up assessment. One adverse event was identified during follow-up and deemed unrelated to the intervention. Evidence was found for intervention effects on both SDQ Total Difficulties and YTP at 12 months (YTP: adjusted mean difference [AMD] = −0.75, 95% confidence interval [CI] = −1.47, −0.03, p = 0.04; SDQ Total Difficulties: AMD = −1.73, 95% CI = −3.47, 0.02, p = 0.05), with stronger effects over 12 months (YTP: AMD = −0.98, 95% CI = −1.51, −0.45, p < 0.001; SDQ Total Difficulties: AMD = −1.23, 95% CI = −2.37, −0.09; p = 0.03). There was also evidence for intervention effects on internalizing symptoms, impairment, perceived stress, and well-being over 12 months. The intervention effect was stable for most outcomes on sensitivity analyses adjusting for missing data; however, for SDQ Total Difficulties and impairment, the effect was slightly attenuated. The per-student cost of delivering the intervention during the trial was $3 United States dollars (USD; or $158 USD per case) and for scaling up the intervention in the modeled scenario was $4 USD (or $23 USD per case). The scaling up cost accounted for 0.4% of the per-student school budget in New Delhi. The main limitations of the study’s methodology were the lack of sample size calculations powered for 12-month follow-up and the absence of cost-effectiveness analyses using the primary outcomes. Conclusions In this study, we observed that a lay counselor–delivered, brief transdiagnostic problem-solving intervention had sustained effects on psychosocial problems and mental health symptoms over the 12-month follow-up period. Scaling up this resource-efficient intervention is an affordable policy goal for improving adolescents’ access to mental health care in low-resource settings. The findings need to be interpreted with caution, as this study was a post hoc extension, and thus, the sample size calculations did not take into account the relatively high attrition rate observed during the long-term follow-up. Trial registration ClinicalTrials.gov NCT03630471.