Background The People’s Planning Campaign (PPC) in the southern Indian state of Kerala started in 1996, following which the state devolved functions, finances, and functionaries to Local Self-Governments (LSGs). The erstwhile National Rural Health Mission (NRHM), subsequently renamed the National Health Mission (NHM) was a large-scale, national architectural health reform launched in 2005. How decentralisation and NRHM interacted and played out at the ground level is understudied. Our study aimed to fill this gap, privileging the voices and perspectives of those directly involved with this history. Methods We employed the Witness Seminar (WS), an oral history technique where witnesses to history together reminisce about historical events and their significance as a matter of public record. Three virtual WS comprised of 23 participants (involved with the PPC, N(R)HM, civil society, and the health department) were held from June to Sept 2021. Inductive thematic analysis of transcripts was carried out by four researchers using ATLAS. ti 9. WS transcripts were analyzed using a realist approach, meaning we identified Contexts, Mechanisms, and Outcomes (CMO) characterising NRHM health reform in the state as they related to decentralised planning. Results Two CMO configurations were identified, In the first one, witnesses reflected that decentralisation reforms empowered LSGs, democratised health planning, brought values alignment among health system actors, and equipped communities with the tools to identify local problems and solutions. Innovation in the health sector by LSGs was nurtured and incentivised with selected programs being scaled up through N(R)HM. The synergy of the decentralised planning process and N(R)HM improved health infrastructure, human resources and quality of care delivered by the state health system. The second configuration suggested that community action for health was reanimated in the context of the emergence of climate change-induced disasters and communicable diseases. In the long run, N(R)HM’s frontline health workers, ASHAs, emerged as leaders in LSGs. Conclusion The synergy between decentralised health planning and N(R)HM has significantly shaped and impacted the health sector, leading to innovative and inclusive programs that respond to local health needs and improved health system infrastructure. However, centralised health planning still belies the ethos and imperative of decentralisation – these contradictions may vex progress going forward and warrant further study. [ABSTRACT FROM AUTHOR]