Accurate and verifiable landholding data are essential for precision agriculture, underpinning equitable water distribution, effective irrigation scheduling, and evidence-based agricultural planning. In many developing contexts, however, legacy land records, customary tenure systems, and fragmented administrative data hinder efficient resource governance. For the first time, we present an innovative, methodical approach to farmer registry modernization—tested and piloted in a mixed farming system characterized by outdated registries and informal tenure arrangements. The approach integrates participatory mapping, mobile-based data collection, and geospatial analysis to generate a technical and socially validated farmer registry. The pilot was conducted in the Kiamanyeki section of Kenya’s Mwea Irrigation Scheme, where 2,397.24 acres were mapped and 997 farmers registered across eight administrative units. Data were collected via KoboCollect and processed via ArcGIS, enabling reconciliation of parcel size discrepancies, boundary visualization, and identification of data gaps associated with absenteeism and nondisclosure. We present a replicable framework that enhances transparency, facilitates stakeholder engagement, and offers a scalable model for improving land information systems in smallholder irrigation contexts. This paper outlines the technical constraints, sociocultural dynamics, and policy pathways for upscaling participatory GISs in support of climate-resilient agricultural governance.