A novel inorganic–organic composite coagulant, poly-ferric-magnesium (PFM) polydimethyldiallylammonium chloride (PDMDAAC), was prepared using FeSO4, MgSO4 and PDMDAAC as raw materials and was introduced to treat landfill leachate. The coagulation performance of the new reagent was evaluated and compared with those of other coagulants. The new reagent was characterized in terms of the analysis of ferron-timed spectroscopy, X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR). The coagulation mechanism was investigated by measuring the ζ-potential reduction and chemical oxygen demand removal at different dosages. Coagulation experiments revealed that the new reagent exhibited better coagulation performance compared with the simple PFM and the PFM + PDMDAAC. Ferron-timed spectroscopy showed that the new reagent exhibited increased effective polymer species concentration. XRD and FTIR spectroscopy showed that the new reagent was not a simple mechanical mixing of PFM and PDMDAAC, but a composite system with inorganic–organic complex interpenetration networks. The predominant coagulation mechanism of the new reagent was charge neutralization at low dosages, as well as adsorption bridging and co-precipitation netting at high dosages, when treating landfill leachate.