Background: High blood pressure (BP) in young adults is an underrecognised public health issue with potential long-term health consequences, including increased mortality risk. To examine the prevalence of prehypertension among US young adults and assess its association with all-cause and cardiovascular mortality. Methods: This cohort study utilised data from the National Health and Nutrition Examination Survey (NHANES) 1999-2016. Cox proportional hazards (PHs) regression models were used to estimate hazard ratios (HRs) for mortality. The Fine and Gray subdistribution hazard model was applied to account for competing risks in cardiovascular mortality. Population attributable fractions (PAFs) were calculated to assess the mortality burden associated with prehypertension and hypertension in this population. Prehypertension was defined as a systolic BP (SBP) of 120-139 mm Hg or a diastolic BP (DBP) of 80-89 mm Hg. All-cause and cardiovascular mortality were ascertained through linkage to the National Death Index (NDI) (up to 2019). Results: Among 18,271 participants (mean [SE] age, 28.6 [0.1] years; 49.6% female), the design-weighted prevalence was 23.9% for prehypertension and 14.3% for hypertension. Compared with normotensive individuals, the adjusted HRs for all-cause mortality were 1.82 (95% CI, 1.20-2.74) for prehypertension and 2.39 (95% CI, 1.50-3.81) for hypertension. The HRs for cardiovascular mortality were 1.37 (95% CI, 0.45-4.15) for prehypertension and 4.17 (95% CI, 1.51-11.51) for hypertension. The PAFs for all-cause mortality were 15.1% for individuals with prehypertension and 16.5% for those with hypertension. For cardiovascular mortality, the PAFs were 6.0% for prehypertension and 36.7% for hypertension. Conclusions: In this nationally representative cohort, prehypertension and hypertension were common among US young adults and associated with increased all-cause mortality. These findings highlight the need for early detection and management of high BP in young adults to reduce mortality risk.