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Complex metal chalcogenides have received growing attention for second-harmonic generation (SHG) activity arising from their noncentrosymmetric structures. In this area, the impact of the Eu(II)-cation on their structures and optical properties has not been well explored. Synthetic investigations of the Eu–Ag–IV-Ch (IV = Sn or Ge; Ch = S or Se) systems have unveiled four Eu(II)-based quaternary chalcogenides exhibiting very large mid-IR SHG responses within the chemically rich systems II–I2-IV-Ch4and II3–I2-IV2–Ch8(Ch = S or Se; I, II, and IV = monovalent, divalent, and tetravalent cations, respectively). Their structures were characterized by single-crystal X-ray diffraction methods to crystallize in noncentrosymmetric space groups, I4̅2mfor EuAg2GeS4(1) and I4̅3dfor Eu3Ag2Ge2Se8(2) Eu3Ag2Sn2Se8(3), and Eu3Ag2Sn2S8(4). The structures consist of body-centered arrangements of (Ge/Sn)Ch4tetrahedra that are fully oriented and bridged by flattened AgCh4tetrahedra into three-dimensional (3D) networks and charge-balanced by Eu(II) cations. Their crystalline powders exhibit mid-IR (2.09 μm) SHG responses among the largest reported to date, ranging from ∼1.9 × AGS (AgGaS2) for 1, to increasing activities of ∼1.9 for 3, ∼4.7 for 2, and ∼7.0 for 4× AGS. Spin-polarized band structure calculations showed that the valence and conduction band edge states stem from interactions of the Ag-to-S/Se-based and Ge/Sn-to-S/Se-based states, respectively, with increasing contributions of the Eu(II) 4f7-based states in the order of 1< 3< 2< 4. Interestingly, this trend correlates with the SHG activity, suggesting a potential new relationship for understanding and attaining cutting-edge SHG properties within Eu(II)-based chalcogenides. Thus, this family helps tounveil a deeper understanding of structure–optoelectronic/SHG property relationships.