An ultra-thin and miniaturized 3D resonant structure capable of linear cross-polarization conversion (CPC) and linear-to-circular polarization (LP-to-CP) conversion for wide incidence angles is presented. The designed 3D resonant structure acts as a multifunctional device, achieving a CPC in the frequency range of 10.42–16.72 GHz with an efficiency of more than 90%. Meanwhile, a linear-to-circular polarization conversion in two frequency bands, 9.34–9.64 GHz and 19.14–20.44 GHz, is also numerically demonstrated. In addition, the proposed structure maintains a stable polarization conversion by altering the oblique incidence angles to a maximum value of 45°. The proposed structure is ultra-thin and miniaturized to cell size, corresponding to 0.08λ and 0.15λ with respect to the central frequency of the CPC operational band. Compared with traditional planar polarization converters, the proposed design shows excellent advantages in miniaturization and wide-angle stability.