Modification of graphitic carbon nitride (g-C3N4) photocatalyst with transition metals was achieved with a simple soft-chemical approach using dicyandiamide monomer and metal chloride as precursors, in combination with a thermal-induced polycondensation at 600 °C under nitrogen atmosphere. The resultant organic–inorganic hybrid materials were thoroughly characterized by a variety of techniques, including X-ray diffraction (XRD), UV/Vis spectroscopy, X-ray photoelectron spectroscopy (XPS), N2-sorption, transmission electron microscopy (TEM), photoluminescence (PL), and FTIR. Benzene hydroxylation and styrene epoxidation reactions were employed to evaluate the catalytic/photocatalytic activity of the synthesized g-C3N4-based catalysts. Results showed that Fe- and Cu-modified g-C3N4 were active for the hydroxylation of benzene to phenol using H2O2 under mild conditions. It was also found that g-C3N4 could promote the catalytic epoxidation of styrene using molecular oxygen as the primary oxidant; after modification with Co and Fe, the catalytic performance for styrene epoxidation with O2 could be significantly improved, especially when coupled with visible-light irradiation.