Paddy ecosystems are recognized as critical hotspots for the generation of methylmercury (MeHg), a potent neurotoxin that poses substantial health risks to humans through dietary exposure via contaminated rice grains. Unlike aquatic food chains, where fish consumption is the primary route of MeHg intake, rice-based diets represent a major but often overlooked pathway in many regions, particularly in Hg-contaminated areas in China. Despite decades of research, several key scientific questions persist, notably the precise provenance of MeHg in rice plants and the prevailing mechanisms that govern Hg transformation within paddy water columns. In recent years, Hg stable isotope techniques, particularly odd-Mass Independent Fractionation (odd-MIF), have provided a unique research perspective for addressing the aforementioned issues. In this study, we compiled a comprehensive database of previously reported MeHg isotope end-member values derived from paddy soil, various rice tissues (roots, stems, leaves, and grains), and paddy water samples collected using the Diffusive Gradients in Thin films (DGT) technique, which provides time-integrated in situ pre-concentration of soluble MeHg. Using a mixing model, we then quantitatively constrained the source contributions of MeHg from different end-members to rice plants. Our modeling results unequivocally demonstrate that MeHg in rice plants originates predominantly from the soil-porewater system, contributing from 72% to 97% of the total MeHg burden, whereas the overlying water plays only a minor role, accounting for merely from 3% to 28%. This finding highlights the critical importance of managing soil biogeochemical conditions to mitigate MeHg transfer into rice plants. Furthermore, according to the Δ¹⁹⁹Hg/Δ²⁰¹Hg slope of MeHg in paddy water captured by DGT and the theoretical parameters of Hg photochemical transformation (the slope is approximately 1.36 for MeHg photodegradation and approximately 1.00 for Hg(II) photoreduction), we quantitatively estimated that photoreduction accounts for about 75% of the photochemical reactions, while photodegradation accounts for about 25%. This indicates that photoreduction is the dominant process driving Hg transformation in paddy water. Overall, this study couples the DGT technique with Hg stable isotopes, providing a novel methodological paradigm and offering new perspectives for a deeper understanding of the formation, transport, and fate of MeHg in aquatic ecosystems. Finally, we emphasize that future efforts should be directed toward developing robust pre-concentration methods and isotopic analytical techniques for ultra-trace MeHg in aquatic systems, thereby enabling a more direct elucidation of its sources, transport, and transformation dynamics within aquatic ecosystems.