Density functional theory (DFT) simulations are performed to explore the physical features of Dy₂MgS₄ and Er₂MgS₄. Local density approximation with Hubbard potential (LDA + U) functional is utilized to examine the magnetic and electronic features. Half metallic ferromagnetic (HMF) behavior is confirmed by the spin‐resolved density of states (DOS) and band structure (BS) plots. Spin‐resolved BS and DOS show that Dy₂MgS₄ and Er₂MgS₄ for spin‐down are metallic, whereas both compounds in spin‐up show semiconductor nature with the bandgaps of 1.731 and 3.081 eV for Dy₂MgS₄ and Er₂MgS₄, correspondingly. Both compounds also feature exchange energies and crystal field splitting, indicating the magnetic characteristics. The estimated magnetic moments of X₂MgS₄ (X = Dy, Er) are 20μ_B and 12μ_B, correspondingly, also revealed by spin‐polarized magnetic density. Optical properties such as refractive index n(ω), absorption coefficient α(ω), dielectric constant , reflectivity R(, extinction coefficient k(ω), and optical conductivity σ(ω) are also explored. The optical bandgaps for MgX₂S₄ (X = Dy, Er) are 1.70 and 1.51 eV, respectively; it shows that both are transparent to visible light. Results indicate that both materials can be suitable for spintronic and solar‐cell usage.