BACKGROUND & AIMS: The impact of serum Omega-6 fatty acids on cardiovascular health is debated, with evidence supporting both protective and harmful effects. To investigate the association between serum Omega-6 fatty acid and mortality from all causes and cardiovascular disease (CVD), utilizing advanced statistical methodologies including competing risk models and multivariate Mendelian randomization. METHODS: Data of 5070 participants from National Health and Nutrition Examination Survey (NHANES) in 2011-2014 wave were analyzed, with follow-up data on mortality sourced from the National Death Index. Serum Omega-6 fatty acids level was measured at baseline. Cox proportional hazards regression and competing risks models was used to estimate hazard ratios (HRs) and 95 % confidence intervals (CIs) for all-cause and CVD mortality by baseline Omega-6 fatty acids level. Restricted cubic splines were used to explore the nonlinearity. Mendelian randomization to assess the causal relationships between Omega-6 levels and mortality. RESULTS: 438 all-cause deaths and 137 CVD deaths were observed during an 83 months median follow-up. Restricted cubic spline analysis demonstrated a U-shaped correlation between baseline serum Omega-6 fatty acid levels with all-cause and CVD mortality risks. Subgroup analysis indicated that for the low-level Omega-6 fatty acid participants, the hazard ratios were 0.68 (95 % CI, 0.55-0.85) for all-cause mortality and 0.62 (95 % CI, 0.40-0.95) for CVD mortality. Conversely, for the high-level participants, the hazard ratios were 1.14 (95 % CI, 1.01-1.28) for all-cause mortality and 1.23 (95 % CI, 1.05-1.44) for CVD mortality. Both univariate and multivariate Mendelian randomization analyses confirmed a positive causal relationship between higher serum Omega-6 fatty acid levels and increased CVD mortality risk. CONCLUSIONS: The findings suggest a U-shaped relationship between serum Omega-6 fatty acid levels and mortality risks, with elevated levels linked causally to increased CVD mortality. These results underscore the need for balanced dietary Omega-6 fatty acid intake to optimize cardiovascular health.