The study examines the association between sodium and potassium intake, the sodium-to-potassium ratio, and all-cause mortality using data from the National Health and Nutrition Examination Survey (NHANES) 2003–2018. The analysis included 13,855 participants aged 40–80 years, with a mean follow-up of 99.395 months. The results show that both daily sodium and potassium intake are inversely associated with all-cause mortality. Restricted cubic spline analyses revealed non-linear relationships, with inflection points at 3133 mg/d for sodium intake, 3501 mg/d for potassium intake, and 1.203 mg/mg/d for the sodium-to-potassium ratio. Subgroup analyses indicated significant interactions between age and sodium intake, with a U-shaped relationship in the elderly population. The study concludes that a healthy diet should include reasonable sodium intake and an appropriate sodium-to-potassium ratio to reduce all-cause mortality.The study examines the association between sodium and potassium intake, the sodium-to-potassium ratio, and all-cause mortality using data from the National Health and Nutrition Examination Survey (NHANES) 2003–2018. The analysis included 13,855 participants aged 40–80 years, with a mean follow-up of 99.395 months. The results show that both daily sodium and potassium intake are inversely associated with all-cause mortality. Restricted cubic spline analyses revealed non-linear relationships, with inflection points at 3133 mg/d for sodium intake, 3501 mg/d for potassium intake, and 1.203 mg/mg/d for the sodium-to-potassium ratio. Subgroup analyses indicated significant interactions between age and sodium intake, with a U-shaped relationship in the elderly population. The study concludes that a healthy diet should include reasonable sodium intake and an appropriate sodium-to-potassium ratio to reduce all-cause mortality.