Crystalline materials with intrinsically low lattice thermal conductivity (κlat) pave the way towards high performance in various energy applications, including thermoelectrics. Here we demonstrate a strategy to realize ultralow κlat using mixed-anion compounds. Our calculations reveal that locally distorted structures in chalcohalides MnPnS2Cl (Pn = Sb, Bi) derives a bonding heterogeneity, which in turn causes a peak splitting of the phonon density of states. This splitting induces a large amount of scattering phase space. Consequently, κlat of MnPnS2Cl is significantly lower than that of a single-anion sulfide CuTaS3 with a similar crystal structure. Experimental κlat of MnPnS2Cl takes an ultralow value of about 0.5 W m−1 K−1 at 300 K. Our findings will encourage the exploration of thermal transport in mixed-anion compounds, which remain a vast unexplored space, especially regarding unexpectedly low κlat in lightweight materials derived from the bonding heterogeneity.