Alignment of doubly degenerate p_xy and nondegenerate p_z band orbitals at the Γ point in the Brillouin zone for high orbital degeneracy has been demonstrated as an effective strategy for advancing thermoelectric AB₂C₂ Zintl compounds in p-type conduction. Formation of solid solutions of different compounds with opposite energy offsets between p_xy and p_z orbitals would enable a minimization of energy offset to realize an orbital alignment, and thereby lead to an enhanced electronic performance. Here we show that alloying EuCd₂Sb₂ with EuZn₂Sb₂ not only effectively aligns the p_xy and p_z valence bands for superior electronic performance, but also significantly reduces the lattice thermal conductivity resulting from the phonon scattering by alloy defects. Eventually, the average thermoelectric figure of merit (zT) of the EuCd_1.4Zn_0.6Sb₂ alloy is enhanced by at least 35% compared to that of pristine EuCd₂Sb₂ and the alloying agent EuZn₂Sb₂, demonstrating that this alloy is a promising thermoelectric material.