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The secreted protein Cystatin C (CyC) is a cysteine protease inhibitor of incompletely characterized biomedical function, used clinically for estimation of glomerular filtration rate. Plasma CyC is elevated in many patients, especially when they receive glucocorticoid (GC) treatment. Here we empirically connect GCs with systemic regulation of CyC. First, we leveraged genome-wide association and structural equation modeling to determine the genetics of the latent trait CyC production in UK Biobank. Using multi-modal genomic, transcriptional, and experimental approaches, we demonstrated that CyC is a direct target of GC receptor, with GC-responsive CyC secretion exhibited by macrophages and cancer cells in vitro. Elevated serum CyC levels were positively correlated with GC levels in a murine model of cancer progression. Consistent with the coupling of CyC levels to GC signaling in a disease relevant manner, CyC predicted elevated all-cause and cancer-specific mortality in humans. These associations were orthogonally confirmed by a polygenic score (PGS) capturing germline predisposition to CyC production. This PGS predicted checkpoint immunotherapy failure in a combined clinical trial cohort of 685 metastatic cancer patients, with available germline exome sequencing. Taken together, our results demonstrate that CyC captures biomedically-relevant variations in endogenous GC activity, raising the possibility that CyC may be a direct effector of GC-induced immunosuppression and therefore a target for combination cancer immunotherapy.