Post-harvest losses (PHL) remain a significant constraint to Nigeria's food security, farm profitability, and sustainable value chain development. This study examined the extent of PHL and adoption of mitigation technologies among smallholder beneficiaries of the Agricultural Transformation Agenda Support Program Phase 1 (ATASP-1). Funded by AfDB and launched in 2015, ATASP-1 operates in four staple crop processing zones across seven states, covering over 200 rural communities. Its objectives are to target rice, cassava, and sorghum to reduce poverty, create jobs, enhance food and nutrition security and raise incomes through sustainable agricultural growth and diversification. Using a multi-stage sampling technique, data were collected from 600 respondents (480 farmers and 120 processors) and analysed with descriptive statistics. Results revealed that ATASP-1 interventions reduced PHL in cassava (48.44%) and sorghum (29.93%) relative to national averages, while rice showed a moderate reduction (22.86%). Adoption of technologies was high, including metallic silos (85%), mud silos (90%), threshers (68%), drying slabs (84%), root choppers (91%) and safety/quality management systems (82%). Key constraints included high machinery costs (78%), limited accessibility (68%), poor infrastructure (59%), unreliable power supply (66%), high transport costs (81%) and high labour costs (73%). The study concludes that ATASP-1 substantially reduced PHL; sustaining and scaling these outcomes requires robust institutional frameworks, capacity building for farmers, and strategic rural infrastructure investment. Additionally, context-specific, zone-based interventions are essential to address heterogeneous constraints and optimise post-harvest loss reduction strategies across value chains.