We report the syntheses and crystal structures of the cluster compounds Tl2Mo15S19 and Ag3Tl2Mo15S19, two novel members of the series M2Mo15X19 (M = In, Tl, K, Rb, Cs, Ba; X = S, Se). While only single crystals of Tl2Mo15S19 were obtained, Ag3Tl2Mo15S19 powders were synthesized, allowing for a study of its thermoelectric properties. Their structures contain Mo6S8iS6a and Mo9S11iS6a units separated by Tl+ cations. Silver insertion increases the electron transfer towards the Mo clusters leading to modifications of the Mo-Mo distances. Electrical resistivity measurements on Tl2Mo15S19 evidence its metallic character. Inserting Ag drives the transport properties towards a semiconducting state characterized by relatively low electrical resistivity and thermopower values. The large, complex unit cell of these compounds results in very low lattice thermal conductivity values below 1 Wm−1 K−1. Although the Ag insertion improves the thermoelectric performances, the ZT value remains moderate with a maximum value of 0.2 at 800 K.