Two acentric chalcogenide compounds, KCdGeS and KCdGeSe, were synthesized via conventional high-temperature solid-state reactions. The crystal structures of KCdGeS and KCdGeS were accurately determined by single-crystal X-ray diffraction and crystallize in the KCdGeS structure type. KCdGeS is isostructural to KCdGeS with superior nonlinear optical properties. For the second harmonic generation (SHG) response, KCdGeS is 18× KFeGeS for samples of particle size of 38–55 μm. The superior nonlinear optical properties of KCdGeS over KFeGeS are mainly contributed by the chemical characteristics of Cd compared with Fe, which are elucidated by nonlinear optical property measurements, electronic structure calculations, and density functional theory calculations. The [CdS] tetrahedra within KCdGeS exhibit a higher degree of distortion and larger volume compared to the [FeS4] tetrahedra in KFeGeS. This study possesses a good platform to investigate how d-block elements contribute to the SHG response. The fully occupied d10-elements are better for SHG susceptibility than d6-elements in this study. KCdGeS is a good candidate as an infrared nonlinear optical material of high SHG response (2.1× AgGaS, samples of particle size of 200–250 μm), type-I phase-matching capability, high laser damage threshold (6.2× AgGaS), and good stability.