We compared the incidence and 30-day mortality of myocardial infarction (MI) in adults with congenital heart disease (CHD) relative to the general population. This cohort study used nationwide population-based medical databases to identify individuals born before 1982 and diagnosed with CHD in Denmark between 1963 and 2012. Patients were followed for first-time MI using data from the Danish National Registry of Patients. For each subject with CHD, we identified 10 controls from the general population, matched by sex and birth year. A unique personal identifier enabled follow-up for migration, death, or MI. We computed cumulative incidences and hazard ratios (HR) adjusted for birth year and sex for MI and 30-day mortality after MI. We identified 10,501 CHD adults alive at 30 years. By 70 years of age, the cumulative incidence of MI was 10% versus 6.5% for controls. The overall HR of MI in subjects with CHD compared with controls was 2.0 (95% CI 1.7 to 2.3). The 30-day mortality was 18% for the 296 subjects with CHD experiencing an MI during follow-up. The overall HR comparing 30-day mortality after MI between subjects with CHD and controls was 1.4 (95% CI 1.0 to 1.8). The greatest mortality was observed in adults with severe CHD (HR 2.7 [95% CI 1.5 to 5.0]). In conclusion, the incidence of MI and the 30-day mortality after MI for severe CHD were increased in adults with CHD compared with the general population. Underlying mechanisms need to be clarified.