The design of new complex mixed metal tellurides (containing low toxicity cations) with intrinsic ultralow thermal conductivity is of paramount importance in the field of thermoelectrics. Herein, we report the synthesis and characterization of polycrystalline and single crystals of a new mixed-metal quaternary telluride Mn_1.8(1)In_0.8(1)Si₂Te₆. The structural aspects and chemical formula of this phase at room temperature have been established using single crystal X-ray diffraction and EDX studies. The trigonal centrosymmetric (space group: P3̄1m) structure of the title phase has cell constants of a = b = 7.0483(7) Å and c = 7.1277(8) Å. The structure has three independent cationic sites, one mixed (In1/Mn1), one partially filled Mn2, and one Si1, which are bonded with Te1 atoms. Each metal atom (In and Mn) in the structure is octahedrally coordinated with six neighboring Te1 atoms. The structure also features dimers of Si atoms, and each Si atom is bonded to three Te1 atoms to form ethane-like Si₂Te₆ units. The optical absorption study of a polycrystalline Mn_1.8In_0.8Si₂Te₆ sample shows a narrow optical bandgap of 0.6(2) eV. Temperature-dependent resistivity and Seebeck coefficient studies confirmed the p-type semiconducting nature of the sample with high values of S (301 μV K⁻¹ to 444 μV K⁻¹). The total thermal conductivity (k_tot) study of the polycrystalline sample shows a decreasing trend on heating with an extremely low value of 0.28 W m⁻¹ K⁻¹ at 773 K. Magnetic measurements indicate a glassy magnetic behavior for the sample below 8 K.