Aim The aim of this study was to examine mortality in patients with both type 1 diabetes (T1D) and coeliac disease (CD). Methods Between 1969 and 2008, we identified individuals with CD through biopsy reports from all pathology departments (n = 28) in Sweden. T1D was defined as a diagnosis of diabetes recorded in the Swedish National Patient Register between 1964 and 2009 in individuals aged ≤30 years. During follow‐up, we identified 960 patients with both T1D and CD. For each individual with T1D and CD, we selected up to five subjects with T1D alone (i.e. no CD), matched for sex, age and calendar period of diagnosis, as the reference group (n = 4608). Using a stratified Cox regression analysis with CD as a time‐dependent covariate, we estimated the risk of death in patients with both T1D and CD compared with those with T1D alone. Results Stratifying for time since CD diagnosis, CD was not a risk factor for death in patients with T1D during the first 5 years after CD diagnosis hazard ratio (HR) 0.87, 95% confidence interval (CI) 0.43–1.73, but thereafter the HR for mortality increased as a function of follow‐up time (5 to <10 years, HR 1.44, 95% CI 0.74–2.79; 10 to <15 years, HR 1.88, 95% CI 0.81–4.36). Having a CD diagnosis for ≥15 years was associated with a 2.80‐fold increased risk of death in individuals with T1D (95% CI 1.28–6.12). Conclusion A diagnosis of CD for ≥15 years increases the risk of death in patients with T1D.