Structural, elastic, electronic and optical features of zinc-magnesium-chalcogenide ternary alloys MgxZn1-xS, MgxZn1-xSe and MgxZn1-xTe have been explored theoretically employing density functional theory based FP-LAPW approach. Structural features have been computed with Wu-Cohen (WC)-GGA functional and found that lattice constant increases, while bulk modulus decreases nonlinearly with increase in Mg-concentration x in each alloy system. Each of the elastic constants C-11, C-12 and C-44 for the cubic specimens decreases with increase in Mg-concentration x in each alloy system. Moreover, each specimen shows elastic anisotropy and found to be ductile in nature. The electronic properties are explored with the modified Becke-Johnson (mBJ), Engel-Vosko (EV)GGA and Perdew-Burke-Ernzerhof (PBE)-GGA functional and found that each binary and ternary sample is a direct band gap (Gamma-Gamma) semiconductor. Band gap for each alloy system increases nonlinearly with Mg-concentration x. Chemical bonding between magnesium and chalcogen are ionic, while that between zinc and chalcogen are covalent in nature. In case of optical transitions, the chalcogen-p of valence band as initial states and Mg-4s, 3p as well as Zn-5s, 4p of conduction band as final states play the dominant role. In each alloy system, nature of variation of each of the static dielectric constant, static refractive index and static reflectivity with Mg-concentration xis opposite, while that of critical point in each of the epsilon(2)(omega), k(omega), sigma(omega)and alpha(omega)spectra with Mg-concentration x is similar to the nature of variation of band gap with Mg-concentration x. Several calculated properties are found to agree well with the corresponding experimental findings.