We present the thermoelectric properties of the quaternary telluride series Tl10–x–ySnxBiyTe6 including Seebeck coefficient (S), electrical conductivity (σ), and thermal conductivity (κ). Three different Tl concentrations, namely 9, 8.67, and 8.33 Tl per formula unit, were selected followed by variations of the Sn:Bi ratio to tune the properties at each Tl concentration. Additionally, crystal structure data and electronic structure calculations were used to model the data and support the findings. The Tl9(Sn, Bi)Te6 system was found to have the highest power factor (S2σ) displaying a value of 8.1 μW·cm–1·K–2 at 587 K, realized with Tl9Sn0.2Bi0.8Te6. Tl8.67Sn0.50Bi0.83Te6 however, displays values closer to 4 μW·cm–1·K–2 at comparable temperatures. Ultimately, the thermoelectric Figure‐of‐merit was determined to reach a competitive 0.6 at 575 K and 525 K for Tl8.33Sn1.12Bi0.55Te6 and Tl8.67Sn0.50Bi0.83Te6, respectively, thereby outperforming the ternary variants Tl10–xSnxTe6.