Myocardial extracellular volume fraction (ECV) reflecting diffuse myocardial fibrosis can be measured with T1 mapping cardiovascular magnetic resonance (CMR) before and after the application of a gadolinium-based extracellular contrast agent. The equilibrium between blood and myocardium contrast concentration required for ECV measurements can be obtained with a primed contrast infusion (equilibrium contrast-CMR). We hypothesized that equilibrium can also be achieved with a single contrast bolus to accurately measure diffuse myocardial fibrosis in patients with congenital heart disease (CHD). Healthy controls (n = 17; median age 24.0 years) and patients with CHD (n = 19; 25.0 years) were prospectively enrolled. Using modified Look-Locker inversion recovery T1 mapping before, 15 min after bolus injection, and during constant infusion of gadolinium-DOTA, T1 values were obtained for blood pool and myocardium of the left ventricle (LV), the interventricular septum (IVS), and the right ventricle (RV) in a single midventricular plane in short axis or in transverse orientation. ECV of LV, IVS and RV by bolus-only and bolus-infusion correlated significantly in CHD patients (r = 0.94, 0.95, and 0.74; p < 0.01, respectively) and healthy controls (r = 0.96, 0.89, and 0.64; p < 0.05, respectively). Bland–Altman plots revealed no significant bias between the techniques for any of the analyzed regions. ECV of LV and RV myocardium measured by bolus-only T1 mapping agrees well with bolus-infusion measurements in patients with CHD. The use of a bolus-only approach facilitates the integration of ECV measurements into existing CMR imaging protocols, allowing for assessment of diffuse myocardial fibrosis in CHD in clinical routine.