In the search of novel photocatalysts to increase the effect of visible light in photocatalysis, g-C3N4 (CN) has become a shining star. Rare earth metals have been used as dopant material to reinforce the photocatalytic activity of CN due to their unique electron configuration recently. In this present study, the pure and different amounts of Ho-doped g-C3N4 (HoCN) photocatalysts were successfully synthesized using urea as a precursor by the one-pot method. Morphological, structural, optical, and vibrational properties of the synthesized photocatalysts were characterized by SEM, EDX, XRD, TGA, XPS, FTIR, PL, TRPL, Raman, DRS, and BET analyses. In addition, theoretical calculations using density functional theory (DFT) were meticulously carried out to delve the changes in the structural and electronic structure of CN with holmium doping. According to calculations, the chemical potential, electrophilicity, and chemical softness are higher for HoCN, while HOMO–LUMO gap, dipole moment, and the chemical hardness are lower for the pure one. Thus, holmium doping becomes desirable with low chemical hardness which indicates more effectivity and smaller HOMO–LUMO gap designate high chemical reactivity. To determine the photocatalytic efficiency of the pure and doped CN photocatalysts, the degradation of methylene blue (MB) was monitored under visible light. The results indicate that holmium doping has improved the photocatalytic activities of CN samples. Most strikingly, this improvement is noticeable for the 0.2 mmol doped CN sample that showed two times better photocatalytic activity than the pure one.