Two new infrared (IR) nonlinear optical (NLO) chalcogenides, SrCdSnQ₄ (Q = S and Se) (1 and 2), were obtained by mixing NLO-active metal chalcogenides and d¹⁰ metal tetrahedral motifs in an alkali metal-containing system. Their structures are constructed with edge- and corner-sharing CdQ₄ and SnQ₄ (Q = S and Se) units to form two-dimensional (CdSnQ₄)²⁻ layers, between which are embedded with Sr²⁺. Compounds 1 and 2 have moderate band gaps of 2.05 and 1.54 eV, respectively. They exhibit strong second-harmonic generation intensities that are comparable to that of the benchmark AgGaS₂ using a 2040 nm laser. Furthermore, their laser-induced damage thresholds are 10 and 5 times higher than that of AgGaS₂, respectively. The strong NLO efficiency of these two compounds can be ascribed to SnQ₄ tetrahedra with a synergetic enhancement in the large distortion of CdQ₄ tetrahedra, shedding light on the exploration of new IR NLO materials with mixed NLO-active and synergetic distorted motifs.