CdS and Cd0.95Mn0.05S were synthesized using a thermolysis procedure in the air and underflow of nitrogen. Rietveld analysis for X-ray diffraction data revealed that all samples have two phases (hexagonal wurtzite and cubic zinc blende). The effect of Mn doping and the preparation conditions on the phase percentages, the crystallite size, lattice parameters and vibrational bands were investigated in detail. The optical band gap of CdS reduced when formed in N2 compared with sample formed in air. Upon doping with Mn, the optical band gap of samples (in air or nitrogen) exhibited a redshift. Density function theory calculation (DFT) revealed a semiconductor feature with a direct band gap for all samples except Mn-doped CdS with cadmium deficiency; it has a half metallic nature. DFT calculations manifested that for the Mn-doped samples, the optical conductivity was enhanced in the visible range considering sulfur deficiency, while considering Cd deficiency the sample has an optical conductivity response only in the IR region. The calculations were performed to examine the influence of Mn doping on the structure, optical, magnetic and electronic characteristics of CdS (stoichiometric and non- stoichiometric).