Photogenerated charge carrier transfer efficiency in graphitic carbon nitride (g-C3N4) based heterostructures depended strongly on the interface nature. In this paper, MoC nanoparticles less than 5 nm were implanted in superior thin g-C3N4 nanosheets to create high charge carrier separation and transfer efficiency. Cubic MoC in N-doped graphitic carbon was first fabricated using dicyandiamide and ammonium molybdate tetrahydrate at 800 °C. The MoC nanoparticles were homogeneously implanted in g-C3N4 nanosheets by further thermal polymerization of bulk g-C3N4 prepared using melamine and MoC/C composites to create a Schottky junction which was confirmed by band gap analysis and free radical capture test. Melamine and dicyandiamide also play important role to create highly efficient catalysts. The MoC nanoparticles revealed fine crystallinity. A well-developed interface between MoC and g-C3N4 was observed. Because of high conductivity of MoC, well-developed interface, and co-catalyst role of MoC, the MoC/g-C3N4 junction exhibited ideal photocatalytic activity for dye degradation and water splitting. The kinetic constant rate of Rhodamine B degradation using a MoC/g-C3N4 sample created by optimized conditions was 0.061 min−1 which was 8.7 times of that of pristine g-C3N4 nanosheets. In the case of without co-catalyst addition, the photocatalytic hydrogen generation rate using this composite sample was 233.0 μmol g−1 h−1 which was 15.2 times of that of initial g-C3N4 nanosheets. These results supply an efficient approach for the first time to homogeneously distribute small cubic MoC nanoparticles in superior thin g-C3N4 nanosheets to construct heterostructures and greatly reduce photogenerated charge carrier recombination.