The structural, electronic, optical, and magnetic properties of the ternary mixed compound CsTexS1−x (x = 0.25, 0.50, and 0.75) with supercell 2 × 2 × 1-CsCl structure have been investigated using a first-principles method, considering that their pure binary compounds, such as CsS and CsTe, crystallize in CsCl phase. The main outcome of the study is that the lattice constants and energy bandgap for the majority spin all exhibit quasilinear variation with increase in the concentration of S ions. It was also determined that the magnetic moments per formula unit are mainly due to the magnetic moments of the p-orbitals of the S/Te anions. Interestingly, the ferromagnetism of the anion is found without the inclusion of transition-metal elements, representing a remarkable property of these materials. Furthermore, the optical properties of these magnetic systems show a quite similar tendency, and the high reflectivity at zero photon energy illustrates weak absorption for all the studied materials. It is therefore suggested that CsS and CsTe could be potential candidates for use in spintronic applications.