Double perovskites have gathered momentous attention due to their structural, optoelectronics, and thermal properties. FP-LAPW + lo technique was employed along with the PBEsol-GGA and TB-mBJ potential. The optimized parameters and E-V parabolic curve were computed employing Birch Murnaghan's equation of state (BM-EOS). The negative formation energy Ef for Rb2YCuCl6 and Cs2YCuCl6 confirms the thermodynamic stability of these double perovskites. The mechanical stability of both compounds was confirmed by their elastic constants Cij, Pugh's ratio, and anisotropy. The indirect band plots of Rb2YCuCl6 (2.57eV) and Cs2YCuCl6 (2.39eV) double perovskites confirm the semiconductor nature. The optical properties like dielectric function, reflectivity, optical conductivity, absorption coefficient, and energy loss function were determined within an energy range of up to 18 eV. The high absorption spectra for the under-study compounds 147.21 × 104 cm−1 at 14.60 eV for Rb2YCuCl6 and 261.30 × 104 cm−1 at 15.57 eV for Cs2YCuCl6 lies in the far UV region determines its potential use in the high-frequency devices. The thermoelectric properties such as the Seebeck coefficient, ZT, and power factor, are investigated using the semi-classical Boltzmann theory implemented in the BoltzTrap code. The peak value of ZT for Rb2YCuCl6 is 0.38 and for Cs2YCuCl6 is 0.33. The suggested findings indicate that Rb2YCuCl6 and Cs2YCuCl6 are potential candidates for thermoelectric and photovoltaic applications.