BACKGROUND: The majority of post-neonatal deaths in children under 5 are due to malaria, diarrhoea and pneumonia (MDP). The WHO recommends integrated community case management (iCCM) of these conditions using community-based health workers (CHW). However iCCM programmes have suffered from poor implementation and mixed outcomes. We designed and evaluated a technology-based (mHealth) intervention package 'inSCALE' (Innovations At Scale For Community Access and Lasting Effects) to support iCCM programmes and increase appropriate treatment coverage for children with MDP. METHODS: This superiority cluster randomised controlled trial allocated all 12 districts in Inhambane Province in Mozambique to receive iCCM only (control) or iCCM plus the inSCALE technology intervention. Population cross-sectional surveys were conducted at baseline and after 18 months of intervention implementation in approximately 500 eligible households in randomly selected communities in all districts including at least one child less than 60 months of age where the main caregiver was available to assess the impact of the intervention on the primary outcome, the coverage of appropriate treatment for malaria, diarrhoea and pneumonia in children 2-59months of age. Secondary outcomes included the proportion of sick children who were taken to the CHW for treatment, validated tool-based CHW motivation and performance scores, prevalence of cases of illness, and a range of secondary household and health worker level outcomes. All statistical models accounted for the clustered study design and variables used to constrain the randomisation. A meta-analysis of the estimated pooled impact of the technology intervention was conducted including results from a sister trial (inSCALE-Uganda). FINDINGS: The study included 2740 eligible children in control arm districts and 2863 children in intervention districts. After 18 months of intervention implementation 68% (69/101) CHWs still had a working inSCALE smartphone and app and 45% (44/101) had uploaded at least one report to their supervising health facility in the last 4 weeks. Coverage of the appropriate treatment of cases of MDP increased by 26% in the intervention arm (adjusted RR 1.26 95% CI 1.12-1.42, p<0.001). The rate of care seeking to the iCCM-trained community health worker increased in the intervention arm (14.4% vs 15.9% in control and intervention arms respectively) but fell short of the significance threshold (adjusted RR 1.63, 95% CI 0.93-2.85, p = 0.085). The prevalence of cases of MDP was 53.5% (1467) and 43.7% (1251) in the control and intervention arms respectively (risk ratio 0.82, 95% CI 0.78-0.87, p<0.001). CHW motivation and knowledge scores did not differ between intervention arms. Across two country trials, the estimated pooled effect of the inSCALE intervention on coverage of appropriate treatment for MDP was RR 1.15 (95% CI 1.08-1.24, p <0.001). INTERPRETATION: The inSCALE intervention led to an improvement in appropriate treatment of common childhood illnesses when delivered at scale in Mozambique. The programme will be rolled out by the ministry of health to the entire national CHW and primary care network in 2022-2023. This study highlights the potential value of a technology intervention aimed at strengthening iCCM systems to address the largest causes of childhood morbidity and mortality in sub-Saharan Africa.