The purpose of this work was to calculate the electronic band structure of ideal and defective Cd1-xMnxS. Ab initio, calculations are performed in the Atomistix Toolkit program within the Density Functional Theory and Local Spin Density Approximation on Double Zeta Double Polarized basis. We have used Hubbard U potential UMn = 3.59 eV for 3d states for Mn atoms. Supercells of 8 and 64 atoms were constructed. After the construction of Cd1-xMnxS (x = 6.25%; 25%) supercells, atom relaxation and optimization of the crystal structure were carried out. Electronic band structure, and density of states were calculated, and total energy have been defined in antiferromagnetic and ferromagnetic phases. Our calculations show that the band gap increases with the increases in Mn ion concentration. It has been established that defects such as interstitial Cd(S) atom, Cd(S) vacancy or Frankel pair in the crystal structure lead to increasing band gap, shifting of Fermi level towards the valence or conduction band.