Abstract Background Cardiac magnetic resonance imaging (CMR) allows for detection of fibrosis in hypertrophic cardiomyopathy (HCM) by quantification of the extracellular volume fraction (ECV). Hypothesis/Objectives To quantify native T1 mapping and ECV in cats. We hypothesize that native T1 mapping and ECV will be significantly increased in HCM cats compared with healthy cats. Animals Seventeen healthy and 12 preclinical HCM, age-matched, client-owned cats. Methods Prospective observational study. Tests performed included indirect blood pressure, CBC, biochemical analysis including total thyroid, urinalysis, transthoracic echocardiogram, and CMR. Cats were considered healthy if all tests were within normal limits and a diagnosis of HCM was determined by the presence of left ventricular concentric hypertrophy ≥6 mm on echocardiography. Results There were statistically significant differences in LV mass (healthy = 5.87 g, HCM = 10.3 g, P < .0001), native T1 mapping (healthy = 1122 ms, HCM = 1209 ms, P = .004), and ECV (healthy = 26.0%, HCM = 32.6%, P < .0001). Variables of diastolic function including deceleration time of early diastolic transmitral flow (DTE), ratio between peak velocity of early diastolic transmitral flow and peak velocity of late diastolic transmitral flow (E : A), and peak velocity of late diastolic transmitral flow (A wave) were significantly correlated with ECV (DTE; r = 0.73 P = .007, E : A; r = −0.75 P = .004, A wave; r = 0.76 P = .004). Conclusions and Clinical Importance Quantitative assessment of cardiac ECV is feasible and can provide additional information not available using echocardiography.