The aim of this study was to determine the association between subclinical hypothyroidism and cardiovascular (CVD) events, and mortality using the atherosclerotic CVD risk score. We carried out an observational study in a prospective cohort that was followed up for 12 years. The study included 3,021 participants aged ≥ 40 years at baseline from the Ansung cohort, part of the Korean Genome and Epidemiology Study. Cox regression models were constructed to evaluate the hazards ratio (HR) and 95% confidence interval (CI) for all-cause mortality and CVD events in groups classified according to thyroid status. Subgroup analysis was performed with a cut-off age of 65 years or 7.5% of the 10-year atherosclerotic CVD risk score. The subclinical hypothyroidism group in the highest quartile of thyroid-stimulating hormone (>6.57 mIU/L) had a significantly increased risk of all-cause mortality (HR 2.12, 95% CI 1.27 to 3.56) and CVD events (HR 1.92, 95% CI 1.21 to 3.04) compared with euthyroid participants. Subgroup analysis by CVD risk revealed that participants with high CVD risk only had a high risk of all-cause mortality (HR 2.18, 95% CI 1.22 to 3.87) and CVD events (HR 2.42, 95% CI 1.35 to 4.33). Further analysis showed that participants aged <65 years with high CVD risk had the highest risk of all-cause mortality (HR 3.50, 95% CI 1.50 to 8.16) and CVD events (HR 3.37, 95% CI 1.46 to 9.57). Our results demonstrated that high thyroid-stimulating hormone levels were associated with a greater risk of mortality and new CVD risks, particularly among subjects with high CVD risk.