Li-based double perovskites (DPs) are extensively studied due to their potential applications for solar cells and thermoelectric devices. In the current paper, I have explored the electronic, optical, transport, and mechanical properties of Li2ScAu (Br/I)6 by density functional theory (DFT) based calculations. The tolerance factor and Born Criteria have been assumed for structural and mechanical stability. In contrast, energy and phonons band structures are examined for thermodynamic and dynamic (Lattice vibration) stabilities. The band gaps of 1.96 eV and 1.42 eV for Li2ScAuBr6 and Li2ScAuI6 fall the absorption bands in the visible zone. The optical characteristics are addressed in the energy range of 0–5.0 eV. The low lattice thermal conductivity, large Seebeck coefficient, and electrical conductivity enhance the figure of merit (ZT) scale that increasse their importance for thermoelectric generators.