We report the Mn doping effect in FeCr₂S₄, which does not possess heterovalency, or static Jahn-Teller distortion, but exhibits large magnetoresistance. In order to see the effects of substituting Fe²⁺ for Mn²⁺ on the conduction, magnetism, and magnetoresistance, we carried out an experimental study of the electrical and magnetic properties of Fe_0.5Mn_0.5Cr₂S₄. FeCr₂S₄ has a spin-glass phase below 60 K ( = T_g) and a ferrimagnetic phase in the temperature range 60-170 K (T_c = about 170 K). In Fe_0.5Mn_0.5Cr₂S₄, both the spin-glass and ferrimagnetic transitions are observed in lower temperature regions (T_g = 45 K, T_c = 140 K). FeCr₂S₄ is a semiconductor, but exhibits metallic-like behavior in the temperature range 150-170 K. Fe_0.5Mn_0.5Cr₂S₄ does not exhibit such metallic behavior, but anomalous transition of the temperature dependence of resistivity is observed in the temperature range 100-180 K. Fe_0.5Mn_0.5Cr₂S₄ exhibits a larger maximum negative magnetoresistance (MR) than FeCr₂S₄. Thses experimental data can be well explained on the basis of the polaron model.