Significance We find that telomere shortening, which usually accompanies cell division in the course of aging, occurs in cardiomyocytes (CMs) of individuals with genetic hypertrophic cardiomyopathy (HCM) or dilated cardiomyopathy (DCM). HCM and DCM CMs differentiated from human-induced pluripotent stem cells (hiPSCs) also exhibit significant telomere shortening relative to healthy controls. By contrast, no telomere shortening was detected in vascular smooth muscle cells in tissue or hiPSC-derived cells, a cell type that does not express the mutant proteins. Our findings provide evidence for accelerated aging in CMs with familial cardiomyopathy. The potential to monitor the dynamics of telomere attrition in hiPSC-CMs over time will enable future mechanistic studies and screens for novel therapeutic agents to arrest telomere shortening and disease progression.