Lymphocyte-to-C-reactive protein ratio (LCR) has been linked to survival in malignancies. However, most studies are from Asia. The knowledge about the link between LCR levels and risks of all-cause mortality and cause-specific mortality among cancer participants in the US is lacking.Using multivariable Cox proportional hazards regression, we investigated the associations between LCR and mortalities in 1999 cancer participants from the National Health and Nutrition Examination Survey 1999-2008 with mortality follow-up through December 31, 2019.The median follow-up time was 156 months. Cancer participants with low LCR levels were associated with increased risks for all-cause and cancer mortality. Based on the full adjustment model, compared to the lowest LCR tertile, the hazard ratios and 95% confidence interval (HR, 95% CI) of all-cause mortality were 0.75 (0.66-0.87) in the second tertile, 0.60 (0.49-0.72) in the top tertile. The HR of cancer mortality was 0.71 (0.52-0.99) in the second tertile and 0.53 (0.35-0.79) in the top tertile. The link between LCR level and all-cause and cancer mortality remained significant when individuals who died within 2 years of follow-up were excluded.This prospective study provided evidence of inverse associations between LCR levels and all-cause and cause-specific mortalities based on representative noninstitutional US cancer survivors. Integrating LCR assessment in the clinical routine of US cancer patients may aid in identifying cancer individuals at high risk of mortalities.© 2025. The Author(s).