Background: Cardiac involvement represents a major cause of morbidity and mortality in patients with myotonic dystrophy type 1 (DM1) and prevention of sudden cardiac death (SCD) is a central part of patient care. We investigated the natural history of cardiac involvement in patients with DM1 to provide an evidence-based foundation for adjustment of follow-up protocols. Methods: Patients with genetically confirmed DM1 were identified. Data on patient characteristics, performed investigations (12 lead ECG, Holter monitoring and echocardiography), and clinical outcomes were retrospectively collected from electronic health records. Results: We included 195 patients (52% men) with a mean age at baseline evaluation of 41 years (range 14–79). The overall prevalence of cardiac involvement increased from 42% to 66% after a median follow-up of 10.5 years. There was a male predominance for cardiac involvement at end of follow-up (74 vs. 44%, p < 0.001). The most common types of cardiac involvement were conduction abnormalities (48%), arrhythmias (35%), and left ventricular systolic dysfunction (21%). Only 17% of patients reported cardiac symptoms. The standard 12‑lead ECG was the most sensitive diagnostic modality and documented cardiac involvement in 24% at baseline and in 49% at latest follow-up. However, addition of Holter monitoring and echocardiography significantly increased the diagnostic yield with 18 and 13% points at baseline and latest follow-up, respectively. Despite surveillance 35 patients (18%) died during follow-up; seven due to SCD. Conclusions: In patients with DM1 cardiac involvement was highly prevalent and developed during follow-up. These findings justify lifelong follow-up with ECG, Holter, and echocardiography. Clinical perspective: What is new? • After 10 years of follow-up cardiac involvement increased from 42% to 66%. • Conduction abnormalities, arrhythmias, and left ventricular dysfunction are common despite most patients not reporting any symptoms. • Addition of Holter monitoring and echocardiography to the standard 12‑lead ECG increases the diagnostic yield for detecting cardiac involvement. What are the clinical implications? • Systematic screening for cardiac involvement with 12‑lead ECG, Holter monitoring, and echocardiography is justified in patients with myotonic dystrophy type 1. • All-cause mortality remains high despite cardiac screening which underlines the need improved risk stratification and multidisciplinary care.