<p>Lead (Pb) and cadmium (Cd) in soil threaten agroecosystem safety and human health. Due to the high bioavailability and mobility of heavy metals in red soil, the risk is further increased. Biochar is widely used for heavy metal immobilization, but original biochar suffers from inherent limitations. Most existing studies of iron (Fe)–manganese (Mn) modified biochar have not focused on red soil, and the relationship with soil–plant responses remains unclear. This study aimed to evaluate and compare the efficacy of sugarcane bagasse biochar (BC) and Fe–Mn modified sugarcane bagasse biochar (FMBC) for Pb and Cd immobilization in red soil, and clarify their effect on plant uptake. BC and FMBC were applied to Pb and Cd contaminated red soil for pot experiments with pak choi (<i>Brassica rapa chinensis</i>) as an indicator plant. The results showed that both BC and FMBC elevated soil pH, organic matter (SOM), enzyme activities, reduced Pb and Cd bioavailability, and transformed Pb and Cd to stable fractions. FMBC outperformed BC, and the 5% FMBC treatment achieved the optimal remediation efficacy. The 5% FMBC decreased the acid-extractable Pb and Cd by 9.33 and 34.40% compared to control. The plant biomass was significantly increased, Pb and Cd content in edible shoots met the limit standards, and bioconcentration (BCF) and translocation factors (TF) were lowered. Overall, this study establishes theoretical foundations and empirical bases for the application of FMBC to immobilize Pb and Cd in red soils, thereby providing a feasible and eco-friendly approach to support sustainable agriculture and the recycling of agricultural waste.</p>