High blood pressure in young adults is an underrecognized 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. This cohort study utilized data from the National Health and Nutrition Examination Survey 1999–2016. Cox proportional hazards 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 blood pressure (BP) of 120–139 mm Hg or a diastolic BP of 80–89 mm Hg. All-cause and cardiovascular mortality were ascertained through linkage to the National Death Index (up to 2019). 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. 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. High blood pressure, including slightly elevated levels known as prehypertension, is common among young adults in the United States, but its long-term impact on the risk of death is not fully understood. This study examined nationally representative data from 18,271 US adults aged 18 to 39 years, collected between 1999 and 2016, and tracked their survival through 2019. The study found that about 24% of young adults had prehypertension and 14% had hypertension during the pooled 1999–2016 survey period. Compared with young adults with normal BP, those with prehypertension had an 82% higher risk of death from any cause, and those with hypertension had a 139% higher risk. Hypertension was associated with more than four times the risk of death from heart disease or stroke, whereas the corresponding estimate for prehypertension was imprecise. Prehypertension and hypertension accounted for an estimated 15% and 16% of all-cause deaths, respectively, and hypertension accounted for approximately 37% of cardiovascular deaths. These findings suggest that checking and managing blood pressure early in adulthood may be an important step to help prevent premature death.