Thermoelectric properties of Cu4Mn2Te4, which is antiferromagnetic with a Néel temperature TN = 50 K and crystallizes in a spinel-related structure, have been investigated comprehensively here. The phase transition occurring at temperatures 463 and 723 K is studied by high-temperature X-ray diffraction (XRD) and differential scanning calorimetry (DSC), and its effect on thermoelectric properties is examined. Hypothetically Cu4Mn2Te4 is semiconducting according to the formula (Cu+)4(Mn2+)2(Te2-)4, while experimentally it shows p-type metallic conduction behavior, exhibiting electrical conductivity σ = 2500 Ω-1 cm-1 and Seebeck coefficient α = 20 μV K-1 at 325 K. Herein, we show that the carrier concentration and thus the thermoelectric transport properties could be further optimized through adding electron donors such as excess Mn. Discussions are made on the physical parameters contributing to the low thermal conductivity, including Debye temperature, speed of sound, and the Grüneisen parameter. As a result of simultaneously boosted power factor and reduced thermal conductivity, a moderately high zT = 0.65 at 680 K is obtained in an excess Mn\In co-added sample, amounting to 5 times that of the pristine Cu4Mn2Te4. This value ( zT = 0.65) is the best result ever reported for spinel and spinel-related chalcogenides.