Background: Long-term data on the natural disease course of unselected patients with ulcerative colitis (UC) are limited. Aims: To determine the long-term course and prognosis of UC, including patients’ risks of surgery, cancer and mortality, in a population-based cohort followed for over 50 years. Methods: All incident patients with UC diagnosed between 1962 and 1987 in Copenhagen County, Denmark were included in a population-based cohort. We extracted information about IBD-related surgeries, cancers and mortality from patient files from 1962 to 1987, and from the Danish National Patient Registry, Cancer Registry, and Register of Causes of Death during 1988-2017. Patients were matched with up to 50 individuals from the general population. Results: We followed 1161 patients for a median of 34 years (range: 0.1-56.0). Median age at diagnosis was 33 years (range: 2-88). The cumulative probability of colectomy 10, 20, 30, 40 and 50 years after diagnosis was 22% (95% CI: 20%-25%), 27% (95% CI: 25%-30%), 31% (95% CI: 28%-34%), 34% (95% CI: 31%-37%), and 40% (95% CI: 36%-44%), respectively. The risk of small intestinal, colon, rectal and anal cancer was higher than among controls, as was cancer of the skin, pancreas and thyroid. All-cause mortality was lower than controls (adjusted RR: 0.90, 95% CI: 0.82-0.99). Conclusion: In this population-based cohort of UC patients diagnosed between 1962 and 1987, 40% underwent colectomy within 50 years of diagnosis. Physicians need to be aware that UC patients are at increased risk of intestinal and extra-intestinal cancers. However, UC patients’ risk of mortality is comparable to that of the background population.