Biomimetic heterogeneous catalysts were prepared by immobilization of meso‐tetrakis(4‐carboxyphenyl)porphyrinatomanganese(III) acetate (MnTCPP) and meso‐tetrakis(4‐carboxyphenyl)porphyrinatoiron(III) chloride (FeTCPP) on graphitic carbon nitride (g‐C3N4) nanosheets. The anchored catalysts were characterized by various techniques such as scanning electron microscopy, thermogravimetric analysis, powder X‐ray diffraction, ultraviolet visible, photoluminescence, flame atomic absorption, and Fourier transform infrared spectroscopy. The thermogravimetric analysis demonstrated that the prepared catalysts were thermally stable up to almost 350°C, exhibiting high thermal stability. In the following, the catalytic efficiency of the prepared nanocatalyst was also investigated for the oxidation of various olefins with hydrogen peroxide (as a green oxidant) and the effect of various parameters which may affect the catalytic efficiency was optimized. The maximum conversion (100% for α‐methylstyrene and 97% for cyclooctene) was obtained in the presence of MnTCPP@C3N4. The Mn porphyrin nanocatalyst shows higher catalytic efficiency compared to the Fe porphyrin.