Thermoelectric colusites, one of the most recently identified and most promising family of complex Cu-S materials, have quickly attracted significant attention based on their outstanding performance. Herein, we investigate the effect of zinc for copper substitution on the thermoelectric properties of the high-performance Cr-Ge colusite, Cu26Cr2Ge6S32. We discuss the striking impact of the aliovalent Zn/Cu substitution on the charge carrier mobility and effective mass and the consequences on the electrical and thermal transport properties. The investigation is supported by first-principles calculations of the electronic density-of-states of doped colusites. The theoretical study reveals the removal of the sharp features at the top of the valence manifold with the incorporation of Zn in the conductive network, with a strong reduction in the estimated relaxation time. These theoretical and experimental observations confirm the importance of disorder within the conductive network and the high sensitivity of the Cu-S tetrahedral framework toward defects in high-performance thermoelectric colusites.