Two new ternary tetrel pnictide phases, Li1.08Ge4.92As7 and Cs0.11Zn0.05Si0.95As, have been synthesized. Both compounds crystallize in new structure types, which demonstrate the flexibility of Ge(Si)–As layers when accommodating electropositive cations of various sizes. Li1.08Ge4.92As7 displays a Ge-As layer motif that has not been observed in ternary Ge-As systems, while Cs0.11Zn0.05Si0.95As exhibits a 3D framework composed of fragments of the 2D parent-phase SiAs. For Li1.08Ge4.92As7, a combination of computational modeling and solid-state 7Li MAS NMR was used to establish the correct crystal structure with one mixed occupied Ge/Li site. The compositions of the phases were confirmed through energy-dispersive X-ray spectroscopy. Transport properties and UV–Vis spectroscopy reveal both reported compounds to be semiconductors. Cs0.11Zn0.05Si0.95As shows high thermal stability with congruent melting at 1300 K and an exceptionally low thermal conductivity of 0.5 W m–1 K–1 at 300 K. Temperature-dependent Li-ion hopping dynamics in Li1.08Ge4.92As7 was characterized using 7Li solid-state NMR.