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.