Density functional calculations of the structural, electronic and optical properties of MgS_xSe_1−x, MgS_xTe_1−x and MgSe_xTe_1−x, in the composition range from x=0 to x=1, are carried out using the highly accurate mBJ potential with the full potential linearized augmented plane waves (FP-LAPW) method. The calculated lattice constants follow Vigard’s law while other properties like bulk moduli, band gaps and optical properties vary nonlinearly with the concentration (x). The binary magnesium chalcogenides, MgS, MgSe and MgTe, are wide and indirect band gap materials, whereas it is found that their ternaries are of direct band gap nature. Indirect band gaps materials are inactive material and could not be used as luminescent materials. The wide and direct band gaps of these alloys predict that they can be efficiently used in optoelectronic devices. Therefore, the optical properties of direct band gap ternary alloys are also calculated.