Cryogenic dielectrics are a crucial component for applied superconducting systems such as high-temperature superconductor (HTS) cables. Here two types of polymer nanocomposites were investigated as dielectrics for cryogenic applications. Both polyimide (PI) and polyamide (PA) nanocomposites showed exceptional performance as dielectrics, with PI as the stronger material. Significant dielectric strength improvement was observed for samples tested in the cryogenic environment when compared to those tested at the room temperature. The PI exhibited a high dielectric strength of 347 ± 67 kV/mm at 92 K, while its nanocomposites were in the range of 261-280 kV/mm. The performance change of these dielectrics was influenced by a number of factors including the density of free charge carriers, localized heat generation and material degradation, thermal contraction of polymers, and polymer-nanoparticle interfacial changes at cryogenic temperatures. The findings from this research can help advance the understanding of breakdown failures in cryogenic dielectrics.