Two new Zintl compounds A10LaCdSb9 (A=Ca, Yb), namely, Ca9.81(1)La0.97(1)Cd1.23(1)Sb9 and Yb9.78(1)La0.97(1)Cd1.24(1)Sb9, have been designed and synthesized by applying the Zintl concept. Although both compounds are isoelectronic with their Ca11InSb9 and Yb11InSb9 analogues, they crystallize in a new structure type with the orthorhombic space group Ibam (No.72) and feature very complex anion structures, which are composed of unique [Cd2Sb6]12− clusters, dumbbell-shaped [Sb2]4− dimers, and isolated [Sb]3− anions. For Yb9.78(1)La0.97(1)Cd1.24(1)Sb9, an extremely low lattice thermal conductivity of 0.29 W m−1 K−1 was observed at 875 K, which almost approaches the lowest reported limit of nonglassy or nonionically conducting bulk materials. According to thermogravimetric (TG) and differential scanning calorimetry (DSC) analyses, both compounds show very good thermal stability and no melting or phase transition processes were found below 1173 K. Although related thermoelectric property studies on Yb9.78(1)La0.97(1)Cd1.24(1)Sb9 only present a maximum ZT of 0.11 at 920 K, owing to its low Seebeck coefficients, these materials are still very promising for their high temperature stability and low thermal conductivity. Furthermore, as mixed cations exist with different charges, it makes this system very flexible in tuning the related electrical properties.