INTRODUCTION: Inflammatory bowel disease (IBD) can lead to mortality directly through severe disease or indirectly via increased risk of malignancy or infection. There are few studies of IBD mortality from the Southern Hemisphere, while new treatments have become available. We aimed to measure 20-year mortality from a population-based IBD cohort in Canterbury, New Zealand. METHODS: A prevalent cohort of 1420 patients with IBD and 600 controls, both recruited in 2005, were followed to 2024. Medical record review identified the underlying cause of death. Standardized mortality ratios compared the deaths in the IBD cohort with those in the general population. Risks were assessed using hazard ratios from Cox regression analysis. RESULTS: During the 23,558 person-years of follow-up, 366 (25.9%) patients with IBD died, at an average age of 75.3 years (SD 13.7). More patients with ulcerative colitis (UC) (n = 195; 29.4%) than Crohn's disease (CD) (n = 162; 22.8%) died. Fewer (18.4%) of the matched control participants died. The SMR for IBD was 1.23 (95% CI 1.11-1.36), with the SMR for CD being 1.42 (1.21-1.65) and UC 1.18 (1.02-1.35). Older age and primary sclerosing cholangitis (PSC) were associated with an increased risk of mortality. Smoking was associated with CD mortality and cancer with UC mortality. Common causes of death included cancer, heart disease, and pneumonia. CONCLUSION: Patients with IBD from this population-based cohort were at increased risk of all-cause mortality over the 20-year follow-up. CD had a higher mortality rate than UC and patients with concomitant PSC were at particular risk.