Cambodia aims to achieve 100 per cent rural sanitation coverage by 2025; however, a concern remains for communities overlooked in “challenging environments”. The Sanitation in Challenging Environments (SCE) Program has advocated for these communities since 2008 through targeted and collaborative projects. Strategic changes made in response to key events resulted in a collective action approach and release of national targets. Additionally, evolution in program management from foreign volunteers to Khmer staff ensured continuity and growth of the program through COVID-19. Using Outcome Harvesting methodology, we conducted a retrospective evaluation of the history of the SCE Program to generate lessons learnt. Analysis revealed insights and systemic challenges for two interrelated themes: communities most overlooked in conventional programming, and localisation of international programs. The SCE Program demonstrates the positive impact and necessity of both localisation and targeted programs for those most overlooked, and we present recommendations for global practitioners working in similar contexts.