The thermoelectric properties of two-dimensional layered ternary compounds AB₂Te₄, in which A (Sn, Pb) and B(Sb, Bi) are group-IV and group-V cations, respectively, were investigated by using first-principles based transport theory. These septuple-atom-layer monolayers have wider band gaps with respect to their bulks, which extend their operating temperature and inhibit the bipolar carrier conduction and thermal conductivity, and more importantly, their energy bands exhibit multiple valence band convergence to a narrow energy range near the Brillouin zone center, which induces an optimal p-type power factor up to 10.94–32.11 W m⁻¹ K⁻² at room temperature. Moreover, these monolayers contain heavy atomic masses and high polarizability of some chemical bonds, leading to small group velocities of phonons and anharmonic phonon behavior that produce an intrinsic lattice thermal conductivity as low as 0.79–3.13 W m⁻¹ K⁻¹ at room temperature. Thus, these monolayers act as p-type thermoelectric materials with thermoelectric figure of merit of up to 2.6–5.5 for SnSb₂Te₄, 0.7–2.2 for PbSb₂Te₄, and 1.6–4.2 for PbBi₂Te₄ in the temperature range of 300 to 750 K, and 4.5–5.9 for SnBi₂Te₄ in the temperature range of 300 to 450 K.