This work reported the structure and optical investigations of CdS_1−xMn_x (x = 0.00, 0.02, 0.04, 0.06, 0.08, 0.10) thin films. The thin films of CdS_1−xMn_x with different compositions were synthesized using electron beam evaporation method under high vacuum conditions at room temperature. X-ray diffraction (XRD) analyses revealed the polycrystalline nature of the deposited films. The observed diffraction peaks were well fitted with a hexagonal crystal structure. In addition, the intensity of XRD peaks decreased by increasing the Mn concentration indicating a monotonical deterioration in the crystalline quality by the doping. The (101) plane is the preferential orientation for the crystal growth of the studied films. The film thickness as well as a refractive index were calculated by Swanepoel’s method. The direct and indirect electronic transitions were found to be responsible for the high absorption process in the studied films. Furthermore, both direct and indirect bandgaps decreased with increasing the Mn content. Other optical parameters such as optical conductivity, real and imaginary parts of dielectric constant, dispersion energy, and dissipation factor were determined and showed a strong dependence on the Mn content. These findings were discussed in terms of the localized states.