Background We implemented a comprehensive intervention (Samata) to address school drop-out and child marriage among rural, marginalised adolescent girls in north Karnataka, south India. Here, we investigate (i) the impact of the intervention on factors hypothesised at baseline to be on the pathway to preventing school drop-out and child marriage, and (ii) associations between these factors and secondary school completion and child marriage. Methods Data was collected for a cluster-RCT evaluation. Factors hypothesised to be on the pathway to improving secondary school retention and delaying age at marriage included: (i) uptake of skills and training by adolescent girls; (ii) uptake of government school scholarships by families of adolescent girls; (iii) gender equitable attitudes among girls; (iv) reduced harassment by boys; and (v) an enabling school environment. Analyses used individual-level cluster-RCT survey data, were intention-to-treat and used mixed-effects logisitic regression models. Results 92.6% (2257/2457) of girls participated at baseline (13-14 years) and 72.8% (1788/2457) participated at end-line (15-16 years). At end-line, uptake of skills and training, gender equitable attitudes around marriage, and recent harassment by boys were significantly higher among girls in the intervention arm but there was no difference in uptake of government school scholarships, gender equitable attitudes around education or eve-teasing, or an enabling school environment by trial arm. Out-of-school/married girls were significantly less likely to have accessed skills or training, to have attended empowerment groups or to have made new friends (past year). They had lower levels of self-efficacy and were twice as likely to report having no hope for the future compared with their in-school/unmarried counterparts. Conclusions Samata was implemented in a context of substantial secular change across India; impacting on some of the factors hypothesised to be on the pathway was not sufficient to improve secondary school retention or delay marriage beyond what was already occurring. School dropout and child marriage were associated with diminished opportunities and well-being among girls. Targeted interventions are still needed; learnings from our study can be used to inform future interventions which similarly aim to impact on child marriage and secondary school retention within programmatic timeframes.