Abstract Aims We examined if a congenital long QT syndrome (cLQTS) diagnosis and severity of cLQTS disease manifestation was associated with increased risk of depression, anxiety, and all-cause mortality. Methods and results All patients with known cLQTS in Denmark were identified using nationwide registries and specialized inherited cardiac disease clinics (1994–2016) and followed for up to 3 years after their cLQTS diagnosis. Risk factors for depression, anxiety, and all-cause mortality were determined using multivariable Cox proportional-hazards regression. An age- and sex-matched control population was identified (matching 1:4). Overall, 589 patients with cLQTS were identified of which 119/589 (20.2%) developed depression or anxiety during follow-up compared with 302/2356 (12.8%) from the control population (P < 0.001). Severity of cLQTS disease manifestation was identified for 324/589 (55%) of patients with cLQTS; 162 were asymptomatic, 119 had ventricular tachycardia (VT)/syncope, and 43 had aborted sudden cardiac death (aSCD). In multivariable models, patients with aSCD, VT/syncope, or unspecified cLQTS disease manifestation had a higher risk of developing depression or anxiety compared with the control population (hazard ratio [HR]=2.4, 95% confidence interval [CI]: 1.1–5.1; HR = 1.9, 95% CI: 1.2–3.0; HR = 1.6, 95% CI: 1.1–2.3, respectively). Asymptomatic patients had similar risk of developing depression or anxiety as the control population (HR = 1.2, 95% CI: 0.8–1.9). During follow-up, 10/589 (1.7%) patients with cLQTS died compared with 27/2356 (1.1%) from the control population (P = 0.5). Furthermore, 4/10 who died had developed depression or anxiety. Conclusion A severe cLQTS disease manifestation was associated with a greater risk of depression or anxiety. All-cause mortality for patients with cLQTS was low.