The electronic structure calculations of defect-free tin monosulfide SnS as well as SnS with existing intrinsic point defects (vacancies in tin (VSn) and sulfur (VS) sublattices), substitutional impurity atoms SbSn and complexes of {VSn–SbSn} type were performed using ab initio density functional theory method in the supercell model. The temperature dependence of stationary photoconductivity and spectral distribution of photosensitivity of SnS crystals doped by antimony were measured in the temperature range of 100–400 K. The results of non-empirical calculations enabled us to analyze the influence of defect formation processes on the macroscopic properties of SnS crystals. It is shown that Sb impurity improves the photoelectric characteristics of SnS crystals.