C-doped, Mo-doped, and (Mo, C)-codoped TiO2 photocatalysts were prepared by a sol-gel process. The photocatalytic activity was evaluated by the photocatalytic degradation of methyl orange (MO) under simulated solar irradiation. Results indicated that both monodoped and codoped TiO2 exhibited better visible light absorption behavior and narrower energy gap than pure TiO2, and codoped TiO2 showed a slightly higher adsorption property in the dark because of higher Brunauer–Emmett–Teller-specific surface area. The photocatalytic activity of monodoped TiO2 was also enhanced, and the (0.04% Mo, C)-codoped sample had the best photocatalytic activity for degrading MO among all of the samples. The reason can be ascribed to the synergistic effect due to Mo and C doping. Furthermore, the transfer pathways of photoinduced carriers and photocatalytic reaction mechanism of (Mo, C)-codoped TiO2 was first investigated.