The structural, elastic, electronic and optical properties of Zn1-xMgxSe ternary mixed crystals are investigated by utilizing the first-principles plane-wave pseudopotential method within the LDA approximations. Some basic physical properties, such as lattice constant, bulk modulus, second-order elastic constants (C-ij), Shear modulus, Young's modulus, Poison's ratio, Lame constants and the electronic band structures, are calculated. We have, also, predicted the optical properties such as dielectric functions, refractive index and energy loss function of these ternary mixed crystals. Our results agree well with the available data in the literature.