Background Identifying predictive markers for future atherosclerotic cardiovascular diseases may be helpful to motivate young adults to promote healthy lifestyle. We sought to determine the association between persistently elevated low-density lipoprotein-cholesterol (LDL-C) and/or triglyceride levels and the atherosclerotic cardiovascular diseases risks in young adults. Methods and Results We conducted a nationwide population-based cohort study of 1 887 853 statin-naive adults aged 20 to 39 years, with LDL-C <190 mg/dL, using the Korean National Health Insurance Service database. Persistently elevated LDL-C and triglyceride levels were defined by ≥3 measurements of ≥160 and ≥175 mg/dL, respectively. The primary outcome was a composite of stroke and myocardial infarction. Among the study population, 11 121 (0.59%) and 167 373 (8.87%) had persistently elevated LDL-C and triglycerides, respectively. During a median follow-up of 5.2 years, 2170 and 1537 incidences of stroke (0.16%) and myocardial infarction (0.23%) occurred. Persistently elevated LDL-C levels were significantly associated with increased risks of the primary outcome, with an adjusted hazard ratio (HR) of 1.396 (95% CI, 1.005-1.940). This association was independent of high-density lipoprotein cholesterol. Persistently elevated triglycerides were significantly associated with increased risks of the primary outcome (HR, 1.120; 95% CI, 1.015-1.236), but attenuated after adjustment for high-density lipoprotein cholesterol. Conclusions Persistently elevated LDL-C and triglyceride levels were associated with atherosclerotic cardiovascular diseases risk in young Korean adults without severe hypercholesterolemia. These lipid abnormalities should be considered risk factors in young adults since their effects on lifetime atherosclerotic cardiovascular diseases risk may become more pronounced over the life course.