Acentric structures host numerous important applications such as second harmonic generation, nonreciprocal responses, etc. In this work, a heteroanionic system of LaAsSBr and LaAsSCl exhibits a good example of how the alignment of lone pair electrons affects crystal structure. Noncentrosymmetric (NCS) chalcohalide LaAsSBr, isostructural to PrAsSCl, and centrosymmetric chalcohalide LaAsSCl were successfully synthesized by a salt flux growth method. Crystal structures were determined by single crystal X-ray diffraction. Both compounds contain trigonal pyramidal [AsS] units with stereochemically active lone pairs in As, aligning in the same direction in LaAsSBr and in opposite directions in LaAsSCl, which account for their acentric crystal structure and centrosymmetric structure, respectively. Electron localization function (ELF) calculations confirmed that the alignment of the [As] motifs contributes to the acentric nature of LaAsSBr. LaAsSBr is predicated to be an indirect bandgap semiconductor by theory calculations with a bandgap of 2.27 eV, which is verified by UV-Vis spectroscopy measurements to be 2.8(1) eV. The acentric structural nature of LaAsSBr was demonstrated by a moderate second harmonic generation (SHG) response of 0.23 AgGaS, where LaAsSCl exhibited no response under the same conditions.