Breaking out-of-plane mirror symmetry of hexagonal $M\text{Se}$ ($M=\mathrm{Ga}$, In, Tl) induces a Janus monolayer enabling exceptionally low lattice thermal conductivity (${\kappa }_l$). At 300 K, lattice thermal conductivities of $\mathrm{GaS}{\mathrm{e}}_{0.5}\mathrm{T}{\mathrm{e}}_{0.5}, \mathrm{InS}{\mathrm{e}}_{0.5}\mathrm{T}{\mathrm{e}}_{0.5}$, and $\mathrm{TlS}{\mathrm{e}}_{0.5}\mathrm{T}{\mathrm{e}}_{0.5}$ are 2.39, 0.59, and 0.77 $\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$, respectively, raising the question of why the In-based system has lower ${\kappa }_l$ than the Tl-based one. Notably, the phonon softening across the entire frequency spectrum in $M\mathrm{S}{\mathrm{e}}_{0.5}\mathrm{T}{\mathrm{e}}_{0.5}$ originates from mass anisotropy, which, together with pronounced anharmonic scattering strength (∼25) and strong rattling motions in $\mathrm{InS}{\mathrm{e}}_{0.5}\mathrm{T}{\mathrm{e}}_{0.5}$ and $\mathrm{TlS}{\mathrm{e}}_{0.5}\mathrm{T}{\mathrm{e}}_{0.5}$, fundamentally account for their minimal ${\kappa }_l$. Furthermore, Grüneisen parameters of $\mathrm{GaS}{\mathrm{e}}_{0.5}\mathrm{T}{\mathrm{e}}_{0.5}$ and $\mathrm{TlS}{\mathrm{e}}_{0.5}\mathrm{T}{\mathrm{e}}_{0.5}$ are positive, reflecting lattice expansion in the $ab$ plane, whereas the negative Grüneisen parameter in $\mathrm{InS}{\mathrm{e}}_{0.5}\mathrm{T}{\mathrm{e}}_{0.5}$ arises from lattice contraction, indicating pronounced anharmonicity within $\mathrm{InS}{\mathrm{e}}_{0.5}\mathrm{T}{\mathrm{e}}_{0.5}$. The “mushroom-shaped” electron localization function (ELF) around Te, mainly from nonbonding lone pairs, reveals localized charge distribution. Together with strong anharmonicity, phonon softening, and enhanced phonon scattering caused by structural asymmetry, these effects drive the ultralow ${\kappa }_l$ in $\mathrm{InS}{\mathrm{e}}_{0.5}\mathrm{T}{\mathrm{e}}_{0.5}$ and $\mathrm{TlS}{\mathrm{e}}_{0.5}\mathrm{T}{\mathrm{e}}_{0.5}$. Te substitution combined with spin–orbit coupling (SOC) suppresses the “sombrero-shaped” valence band maxima, producing a parabolic dispersion that lowers hole effective mass and improves charge transport. These results demonstrate that structural asymmetry induced by Te substitution, which breaks the out-of-plane mirror symmetry, plays a pivotal role in improving the thermoelectric performance, with a maximum ZT of 1.47 ($p$-type), 2.41 ($p$-type), and 2.01 ($n$-type) in $\mathrm{GaS}{\mathrm{e}}_{0.5}\mathrm{T}{\mathrm{e}}_{0.5}, \mathrm{InS}{\mathrm{e}}_{0.5}\mathrm{T}{\mathrm{e}}_{0.5}$, and $\mathrm{TlS}{\mathrm{e}}_{0.5}\mathrm{T}{\mathrm{e}}_{0.5}$, respectively, at 800 K.

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• Lattice thermal conductivity
• Thermoelectrics
• Janus materials
• Monolayer films
• Density functional calculations