[Objective] The conversion of farmland to non-agricultural uses poses a threat to food security. However, there is currently few research on the characteristics and causes of farmland conversion in hilly areas. This study takes northeastern Guangdong Province (NGP) as an example to explore the patterns and mechanisms of farmland conversion in hilly regions, providing theoretical guidance for preventing farmland conversion. [Methods] Using land use images from NGP in 1990, 2000, 2010, and 2020, the information on farmland conversion was extracted. Spatial autocorrelation analysis and the optimal parameter geographic detector method were employed to investigate the spatial-temporal distribution and influencing factors of farmland conversion. [Results] (1) From 1990 to 2020, cropland non-agriculturalization in NGP exhibited a trend of "steady development followed by a sharp decline," with a cumulative non-agriculturalized area of 228,537.63 hectares and a non-agriculturalization rate of 25.31%. Forest land and construction land were the primary types of non-agriculturalization. (2) Vertically, cropland above 1000 meters in elevation and with a slope of 25-35° is more prone to non-agriculturalization; horizontally, cropland within 50 meters of construction land and 1000-1500 meters from rivers is more likely to undergo non-agriculturalization. (3) Cropland non-agriculturalization in NGP exhibits significant positive clustering, with Molan’s I gradually decreasing from 0.371 to 0.255. The changes in "high-high" clustering and "low-low" clustering dominated the evolution of the spatial autocorrelation pattern of cropland non-agriculturalization rates. (4) Agricultural population and cropland fragmentation consistently had strong explanatory power for the spatial distribution of cropland non-agriculturalization, while the explanatory power of socio-economic factors for non-agriculturalization has weakened over the past decade. The interaction of multiple factors, especially agricultural and socio-economic factors, can further enhance the explanatory power for cropland non-agriculturalization. [Conclusion] Over the past 30 years, the area and spatial clustering of farmland conversion in NGP have declined. However, the impact of agricultural population and farmland fragmentation on farmland conversion has intensified. It is suggested to reasonably control the speed of urban expansion, consolidate fragmented farmland, introduce small-scale agricultural machinery to improve farming conditions, and encourage farmers to cultivate by promoting rural e-commerce, in order to address the issue of farmland non-agriculturalization.