<p>Broadband near-infrared phosphors with longer wavelengths are urgently needed for applications such as deep-tissue biomedical imaging. However, the detailed mechanism of Mn²⁺-activated NIR emission remains elusive. Guided by the theory of the nephelauxetic effect, sulfide phosphors with strong covalent bonding have become among the most promising alternatives for obtaining Mn²⁺ long-wavelength luminescence. On this basis, the synergistic strategy of combining stronger crystal field effects can make the red-shift emission of isolated Mn²⁺ more reliable. In this work, we studied a series of M(Ga, In)₂S₄:Mn²⁺ (M = Ca, Sr, Ba) phosphors featuring strong crystal field environments, and reported for the first time the luminescence behaviors of Mn²⁺-doped MIn₂S₄:Mn²⁺ (M = Sr, Ba). The preferential occupation behavior of different sites results in significant differences between MGa₂S₄:Mn²⁺ (M = Ca, Sr) and MIn₂S₄:Mn²⁺ (M = Sr, Ba) with the same crystal structure. The severe distortion of the polyhedra further enhances their performance in ultrawideband deep red to near-infrared luminescence (600–850 nm, FWHM ≈ 123–257 nm), which is far superior to that of similar materials. Furthermore, the fabricated PC-LED devices have excellent vascular imaging and plant illumination capabilities. This study provides a new perspective for understanding the near-infrared luminescence of Mn²⁺.</p>