Herein, abundant and underutilized rubber seed shell (RSS) was valorized for one-pot production of multifunctional magnetic biochar (MBC) through one-pot FeCl3 activation. Fe3O4 and Fe0 crystals were formed in MBC, providing a saturation magnetization of 6.83 emu/g. In addition, the material also had a specific surface area of 378 m2/g and a total pore volume of 0.22 cm3/g. MBC was subsequently explored for catalytic ozonation of Ponceau 4R (P4R). As a result, MBC enhanced P4R ozonation in a broad pH range of 3.0–9.0. At pH 5.8, the pseudo-first-order rate constant of P4R decolorization with MBC improved by 50% compared to that without MBC. Summarily, RSS-derived MBC is a potential catalyst for enhanced ozonation of Ponceau 4R thanks to its low cost, eco-friendliness, relative effectiveness, and magnetic separability.