The increasing generation of food and garden organics (FOGO) in municipal waste streams demands effective treatment solutions that support biogas recovery and environmental sustainability. This research investigates the hydrolysis kinetics of FOGO under mesophilic high-solids anaerobic digestion (HSAD) conditions using a modelling-based approach. Five co-digestion batches with varying food-to-garden waste ratios were analysed using first-order kinetics and the Modified Gompertz model. The first-order kinetic model yielded consistent hydrolysis rate constants (0.0744–0.0881 day−1), demonstrating greater accuracy in mixtures with a higher proportion of food waste. However, the model exhibited reduced accuracy in batches with higher proportions of garden waste, attributed to the complex and non-linear degradation behaviour of lignocellulosic components. The Modified Gompertz model provided better predictive performance by effectively capturing lag phases and sequential methane production in both two- and three-phase configurations. Among all batches, FG29 (40% food waste, 60% garden waste) exhibited the most consistent and stable hydrolysis behaviour throughout the digestion process. These results highlight the critical role of substrate composition in governing hydrolysis dynamics and determining the appropriateness of kinetic model selection. The outcomes contribute to improved process design and optimisation of co-digestion systems treating heterogeneous organic waste under high-solids conditions.