Graphitic carbon nitride (g-C3N4) is a visible light active photo-catalyst, but its photo-catalytic efficiency is hampered by rapid recombination of the photo-induced charge carriers in g-C3N4. This adverse charge recombination of g-C3N4 is efficiently retarded by compositing g-C3N4 with cadmium selenide (CdSe). The energy band gap of CdSe is narrower than that of g-C3N4, and also its band positions are compatible with g-C3N4 in order to carry out the lateral intersystem crossing of the photo-induced electrons from the conduction band of g-C3N4 to that of CdSe. In this work, g-C3N4/CdSe nanocomposites with four different CdSe mass loading (1%, 2.5%, 5%, and 10%) were synthesized by pulsed laser induced method, where laser fragmentation (LFL), and laser defect engineering (LDL) are the underlying processes. The structural, morphological, and elemental characterizations using XRD, TEM, FE-SEM, and EDX confirmed the proper anchoring of CdSe nanoparticles on the g-C3N4 polymeric surface. The modified band structure in the g-C3N4/CdSe nanocomposite not only enhanced the visible light absorption, but also effectively retarded the photo-induced charge recombination, which are reflected in the optical absorption spectra, and the photoluminescence spectra respectively. Photo-catalytic degradations of Rhodamine B (RhB), and methylene blue (MB) dyes under visible light in the presence of four different variants of g-C3N4/CdSe nanocomposites were estimated, and bench marked with pure g-C3N4. As anticipated, the photo-catalytic degradation efficiency of all the variants of g-C3N4/CdSe nanocomposites is significantly higher than that with pure g-C3N4. The highest photo-catalytic degradation rate constant of 0.0222 min−1, 0.0178 min−1 respectively for MB and RhB were achieved using g-C3N4/CdSe nanocomposite with 2.5% CdSe mass content. As a second application, g-C3N4/CdSe nanocomposite was also used in the electrocatalytic hydrogen evolution process in the acidic medium. Thin this study, the observed over potential recorded was lower for g-C3N4/CdSe nanocomposite than that with pure g-C3N4, indicating the higher hydrogen evolution capability of this nanocomposite as the electro-catalyst.