Type 1 diabetes mellitus (DM) is associated with early atherosclerosis and increased cardiovascular mortality. The relation between carotid intima–media thickness (cIMT), a marker of subclinical atherosclerosis, and left ventricular (LV) mass, an independent predictor of cardiovascular morbidity, has not been previously studied in type 1 DM. The Epidemiology of Diabetes Interventions and Complications (EDIC) study is a multicenter observational study designed to follow up the Diabetes Control and Complications Trial (DCCT) cohort. LV mass was measured with cardiac magnetic resonance imaging at EDIC year 15 and common cIMT was assessed using B-mode ultrasound at EDIC year 12. Multivariable linear regression models were used to assess the relation between cIMT at year 12 and LV mass at year 15. In total 889 participants had cardiac magnetic resonance imaging and cIMT measurements available for these analyses. At EDIC year 15, mean age of the participants was 49 ± 7 years, mean DM duration was 28 ± 5 years, and 52% were men. Spearman correlation coefficient (r) between LV mass and cIMT was 0.33 (p <0.0001). After adjusting for basic covariates (machine, reader, age, and gender), a significant association between LV mass and cIMT (estimate 2.0 g/m² per 0.1-mm cIMT increment, p <0.0001) was observed. This association was decreased by the addition of systolic blood pressure, in particular 1.15 g/m² per 0.1-mm cIMT increment (p <0.0001), and to a lesser extent other cardiovascular disease risk factors. Furthermore, the relation observed between LV mass and cIMT was stronger in patients with shorter duration of DM. In conclusion, cIMT was an independent predictor of larger LV mass in a well-characterized population with type 1 DM.