Co_1.2−xMn_xFe_1.8O₄ (0≤x≤0.4) compositions were synthesized by the autocombustion route by keeping the oxidizer to fuel ratio (Φ_e) at 1. Thermogravimetric analysis (TGA) shows the stable phase formation takes place at a temperature above 600 °C. Structural characterization of all the samples was carried out by the x-ray diffraction technique. Room temperature magnetization measurements showed that, for the substitution of Co by Mn, there is an initial increase in the saturation magnetization (M_s) for lower concentrations (i.e. x = 0.1 and 0.2); and then the magnetization decreases for higher concentrations (i.e.  x = 0.3 and 0.4). Also, it is observed that the coercivity (H_c) goes on decreasing with the substitution of Mn content, except for x = 0.3 which shows a slight increase in coercivity as compared to x = 0.4. Room temperature dielectric properties, namely relative dielectric permittivity (ε'), dielectric loss (tanδ) and ac conductivity (σ_ac), for all the samples were studied as a function of applied frequency in the range from 20 Hz to 1 MHz. These studies indicate that the relative dielectric permittivity goes on increasing with the increase of Mn content in Co ferrite and also all the samples show the usual dielectric dispersion which is due to the Maxwell–Wagner-type interfacial polarization. The ac conductivity measurement suggests that the conduction is due to small polaron hopping.