Background/Objectives: Tumor-immune system interactions shape the progression of cutaneous squamous cell carcinoma (cSCC). Serum biomarkers for risk stratification remain limited. Complement factor H (CFH) regulates the alternative complement pathway. It has been linked to immunosuppression and cSCC development in tissue-based studies. We investigated whether serum CFH is associated with tumor aggressiveness and may help predict immunotherapy outcomes in advanced cSCC. Methods: In this retrospective, single-center study, pre-treatment serum CFH levels were measured in 104 cSCC patients (62 high-risk and 42 advanced) using ELISA. Associations with clinical characteristics, disease stage, and response to cemiplimab were analyzed. Subgroup comparisons considered immune status and inflammatory comorbidities. Results: Advanced cSCC patients had significantly higher CFH levels than high-risk patients (OR 0.13, p = 0.026), independent of tumor diameter or invasion depth. Among advanced cSCC cases, lower baseline CFH was associated with more prolonged progression-free survival (median 19.8 vs. 3.07 months, p = 0.029; HR 0.29, p = 0.014), independent of covariates including immunosuppression. CFH levels during therapy did not predict treatment response. ROC analysis showed moderate discriminatory ability with CFH alone (AUC 0.625), which improved when combined with clinical variables in a multivariable risk model (AUC 0.767). Conclusions: Serum CFH is an independent predictor of cemiplimab response and reflects biological aggressiveness in cSCC beyond conventional high-risk features. These findings support the use of CFH in clinical risk models and warrant external validation in multicenter cohorts.