Congenital long QT syndrome (LQTS) is an inherited cardiac channelopathy characterized by prolongation of the QT interval on electrocardiogram (ECG), and is associated with an increased risk of life-threatening ventricular arrhythmias. Mutations in over a dozen distinct genes have been implicated in the pathogenesis of this group of disorders. LQTS type 1 (LQT1), the most prevalent LQTS subtype, is characterized by a heterozygous loss-of-function mutation in the KCNQ1 gene, which codes for the α-subunit of the delayed rectifier inward potassium ion channel. The association of LQTS with dilated cardiomyopathy (DCM) is rare but has been reported in the presence of sodium channel gene mutations, as seen in LQTS type 3. In this case report, we describe a patient with familial LQT1 (KCNQ1 mutation) identified in infancy who was subsequently diagnosed with severe DCM later in childhood.