${\mathrm{Nd}}^{3+}$ sensitized ${\mathrm{Er}}^{3+}$ : ${}^4{I}_{11/2}\to {}^4{I}_{13/2}$ mid-IR emissions around $2.7\mathrm{\mu m}$ were investigated in the transparent ${\mathrm{Ga}}_2{\mathrm{S}}_3–{\mathrm{GeS}}_2–\mathrm{CsCl}$ chalcohalide glasses for the first time. Remarkably, it is found that ${\mathrm{Nd}}^{3+}$ greatly enhances ${\mathrm{Er}}^{3+}$ $2.7\mathrm{\mu m}$ emission by a maximal 20 times, and depopulates the lower level of ${\mathrm{Er}}^{3+}$ : ${}^4{I}_{13/2}$ for population inversion. Based on Judd–Ofelt theory, the $2.7\mathrm{\mu m}$ emission cross section is calculated (as high as $0.66\times {10}^{-20}{\mathrm{cm}}^2$ ) and the gain property of the ${\mathrm{Er}}^{3+}$ : ${}^4{I}_{11/2}\to {}^4{I}_{13/2}$ transition is discussed. Hopefully, the materials studied here may find potential applications in the fields of fiber amplifiers and mid-IR lasers.