Alignment of valence bands has been demonstrated to be effective in promoting the thermoelectric performance of p-type AB₂X₂ zintl phases. In this work, the degeneracy of the valence bands is manipulated by alloying CaCd₂Sb₂ with CaMg₂Sb₂. It is found that the Γ( p_xy) band and the Γ( p_z) band were effectively converged in CaCd_1.5Mg_0.5Sb₂. By further doping Ag at the Cd site, the carrier concentration can be maintained when the alloying concentration varies. The room-temperature Seebeck coefficient increased from ∼150 μV K⁻¹ in CaCd₂Sb₂ to ∼190 μV K⁻¹ in CaCd_1.5Mg_0.5Sb₂ when the carrier concentration was maintained at ∼2.5 × 10¹⁹ cm⁻³. In addition, Cd/Mg substitutional point defects with substantial atomic mass difference induced significant phonon scattering; thus, a lattice thermal conductivity as low as ∼0.5 W m⁻¹ K⁻¹ was achieved at 750 K. Eventually, a peak zT value of ∼1.3 was realized in CaCd_1.494Ag_0.006Mg_0.5Sb₂.