Motivated by the wide-band-gap semiconductor properties of Zn-monochalcogenides (Zn-X; X:S, Se and Te), especially for their crucial industrial applications, we use a first-principles approach to investigate the B3 (zinc blende type) to B1 (rock salt type) structural transitions in this series of compounds as a function of pressure and temperature. Under static conditions (i.e., T = 0 K), the transition pressure is found to steadily drop from ZnS to ZnTe via intermediate ZnSe. Our calculations within quasi-harmonic approximation yield negative Clapeyron slopes of the B3–B1 phase boundaries for all the three compounds, where ZnTe has the highest negative slopes. We also present a completely new set of calculations for the thermoelasticity of Zn-X phases in the temperature range 0–1100 K. This article then addresses how the B3–B1 phase transitions can influence the mechanical as well as electronic properties of Zn-X. This phase transition always results in a softening of their elastic constant C12; however, C11 and C44 get stiffened. The same structural transition switches a semiconductor to conductor-type electronically favorable transition, as inferred from their high-pressure electronic structure. Among the three Zn-X com-pounds, ZnTe becomes the most metallic phase following the B3–B1 transition. Our findings offer a novel explanation for the complete loss of semiconductor property of these monochalcogenides at elevated pressures.