Elemental doping has already been established to be one of the most effective approaches for band-gap engineering and controlled material response for improved photocatalytic activity. Herein atomically thin ZnIn2S4 (ZIS) nanosheets were doped with O and N separately, and the effects of doping were spectroscopically investigated for photocatalytic H2 evolution. Steady-state photoluminescence studies revealed an enhanced charge-carrier population in the doped systems along with a defect-state-induced broad peak in the red region of the spectra. Transient absorption (TA) spectroscopy demonstrated that the conduction-band-edge electrons are transferred on an ultrafast time scale to the inter-band-gap defect states. TA analysis suggests that O and N doping contributes to the defect state concentration and ensures an enhanced photocatalytic activity of the system. This detailed spectroscopic analysis uncovers the role of inter-band-gap defect states in the photocatalytic activity of ZIS and will open new avenues for the construction of nanosheet-based optical devices.