In this article, we have performed first-principles studies on structural stability, magnetic, electronic, and thermoelectric properties of (K/Rb)2MnF6. The structural optimization shows the ferromagnetic ground state with high Curie temperature (TC) of 609K and 596K for K2MnF6 and Rb2MF6 respectively. Also, the estimated tolerance and octahedral factor demonstrated the cubic phase stability of (K/Rb)2MnF6. In addition, the spin-polarized electronic studies revealed the half-metallic nature of these systems. Further, the spin-based thermoelectric prediction showed the higher figure of merit (ZT) of the down channel originated from the higher Seebeck coefficient with low thermal conductivity of both compounds. Altogether,(K/Rb)2MnF6 are potential candidates for spintronic and spin-based thermoelectric applications.