Aims: Persons with diabetes mellitus have increased all-cause mortality compared with the general population. Nationwide studies on causes of death and mortality among young persons with diabetes mellitus are sparse. The aim of this study was to examine all-cause and cause-specific mortality in children and young adults with and without diabetes. Methods and results: The study population consisted of all persons in Denmark aged 1–35 years in 2000–2009 and 36–49 years in 2007–2009, which equals 27.1 million person-years. All 14,294 deaths in the 10-year period were included and cause of death was established based on information from autopsy reports and death certificates. The Danish Register of Medicinal Product Statistics was used to identify persons with type 1 diabetes and type 2 diabetes. During the study period, which included 153,070 diabetic person-years, 669 (5% of all deceased) persons with diabetes mellitus died, of which 70% had type 1 and 30% had type 2 diabetes. Persons with diabetes mellitus had an all-cause mortality rate (ASMR) of 327 per 100,000 person-years compared with 74 per 100,000 person-years among persons without diabetes mellitus (ASMR ratio 4, p < 0.001). The leading cause of death among persons with diabetes mellitus was cardiac diseases (n = 230, 34%) with an ASMR ratio of 8 (95% confidence interval 6–9). Conclusions: Young persons with diabetes mellitus had four-fold increased all-cause mortality and eight-fold increased cardiovascular mortality compared with persons without diabetes mellitus. Focus on cardiovascular risk monitoring and management among young persons with diabetes mellitus is warranted to prevent premature death in diabetes mellitus.