We evaluated the impact of Samata, a 3-year multilayered intervention among scheduled caste/scheduled tribe (SC/ST) adolescent girls in rural northern Karnataka, on family-level (parents or guardian) attitudes and direct and indirect norms related to child marriage and girl's education. Endline data from 1840 family members were used to assess the effect of Samata on attitudes and norms related to schooling and child marriage, while data from 4097 family members (including 2257 family members at baseline) were used to understand the shifts in attitudes and norms over the period 2014-2017. Overall, we found that the programme had little impact on family-level attitudes and norms. However, there were shifts in some attitudes, norms and perceived sanctions between baseline (when girls were aged 13-14 years) and endline (when girls were aged 15-16 years), with some becoming more progressive (e.g. direct norms related to child marriage) and others more restrictive (e.g. norms around girls completing secondary education and norms related to child marriage and educational drop-out, blaming girls for eve teasing and limiting girls' mobility so as to protect family honour). Moreover, non-progressive norms related to marriage and education were strongly associated with child marriage and secondary school non-completion among adolescent girls in this rural setting. Norms hypothesised to be important for marriage and schooling outcomes were indeed associated with these outcomes, but the intervention was not able to significantly shift these norms. In part, this may have been due to the intervention focusing much of its initial efforts on working with girls alone rather than family members, the relevant reference group. Future interventions that seek to affect norms should conduct formative research to clarify the specific norms affecting the outcome(s) of interest; likewise, programme planners should ensure that all activities engage those most influential in enforcing the norm(s) from the beginning. ClinicalTrials.gov registration number: NCT01996241.