Background Depression is a nontraditional risk factor for cardiovascular disease (CVD). Data on the association of depression and poor mental health with CVD and suboptimal cardiovascular health (CVH) among young adults are limited. Methods and Results We used data from 593 616 young adults (aged 18–49 years) from the 2017 to 2020 Behavioral Risk Factor Surveillance System, a nationally representative survey of noninstitutionalized US adults. Exposures were self‐reported depression and poor mental health days (PMHDs; categorized as 0, 1–13, and 14–30 days of poor mental health in the past 30 days). Outcomes were self‐reported CVD (composite of myocardial infarction, angina, or stroke) and suboptimal CVH (≥2 cardiovascular risk factors: hypertension, hypercholesterolemia, overweight/obesity, smoking, diabetes, physical inactivity, and inadequate fruit and vegetable intake). Using logistic regression, we investigated the association of depression and PMHDs with CVD and suboptimal CVH, adjusting for sociodemographic factors (and cardiovascular risk factors for the CVD outcome). Of the 593 616 participants (mean age, 34.7±9.0 years), the weighted prevalence of depression was 19.6% (95% CI, 19.4–19.8), and the weighted prevalence of CVD was 2.5% (95% CI, 2.4–2.6). People with depression had higher odds of CVD than those without depression (odds ratio [OR], 2.32 [95% CI, 2.13–2.51]). There was a graded association of PMHDs with CVD. Compared with individuals with 0 PMHDs, the odds of CVD in those with 1 to 13 PMHDs and 14 to 30 PHMDs were 1.48 (95% CI, 1.34–1.62) and 2.29 (95% CI, 2.08–2.51), respectively, after adjusting for sociodemographic and cardiovascular risk factors. The associations did not differ significantly by sex or urban/rural status. Individuals with depression had higher odds of suboptimal CVH (OR, 1.79 [95% CI, 1.65–1.95]) compared with those without depression, with a similar graded relationship between PMHDs and suboptimal CVH. Conclusions Depression and poor mental health are associated with premature CVD and suboptimal CVH among young adults. Although this association is likely bidirectional, prioritizing mental health may help reduce CVD risk and improve CVH in young adults.