Green chemistry has fostered research on recyclable, insoluble, and easily separable heterogeneous catalysts. Carbon materials are widely used for renewable energy and environmental studies. Here, we used green Pistachio peel, a biomass waste for the synthesis of magnetic carbon-based solid acid (Fe₃O₄@C-SO₃H) by carbonization and sulfonation. The physicochemical properties of the nanocatalyst were characterized using XRD, FT-IR, FE-SEM, TGA, VSM, and TEM. The catalytic activity of Fe₃O₄@C-SO₃H was investigated in the synthesis of isoxazole-5(4H)-one, 1-amido alkyl-2-naphthol, pyrano[2,3-c]pyrazole, and 2,3-dihydro quinazoline-4(1H)-one derivative, and some of the synthesized compounds were screened for their anti-microbial activity. Furthermore, the recovery and reuse of the catalyst were demonstrated six times without detectible loss inactivity. The concentration of H⁺ loaded on the Fe₃O₄@C-SO₃H was reported to be 1.3 mmol g^-1. The well-defined Fe₃O₄@C-SO₃H core–shell heterostructures exhibited high stability, efficient recyclability (6 cycles).