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Background: Most studies from high income countries consistently report that preexisting diabetes reduces overall survival of cancer patients. We examined this association in a retrospective cohort study using two nation-wide population-based data sets in Latvia. Material and methods: The Cancer Register, linked with the Diabetes Register and Causes of Death Database, was the first data source used to select 22,936 men and 25,338 women with cancer diagnosed from 2009 to 2013. The follow-up period ended on 28 February 2015. The National Health Service data served as a second data source, which was used to select 10,130 men and 13,236 women with cancer as the main diagnosis, who were discharged from oncology hospitals from 2009 to 2012. Prescriptions of reimbursed antidiabetic medications indicated prior diabetes status. The follow-up period started at the date of discharge and ended on 31 December 2013. A Cox proportional hazards model was used to assess association between preexisting diabetes and all-cause mortality, adjusted for age. Results: Men with preexisting diabetes had better overall short-term survival: the age-adjusted hazard ratios (95% CI) were 0.86 (0.79–0.93) for the first year and 0.89 (0.80–0.98) for the first two years after cancer diagnosis according to the disease register and health service data, respectively. After three full follow-up years, their relative mortality increased, with an age-adjusted hazard ratio of 1.60 (1.28–1.99). Among women, preexisting diabetes was associated with slightly higher all-cause mortality during the entire follow-up period, with age-adjusted hazard ratios of 1.17 (1.10–1.24) for the disease register data and 1.11 (1.02–1.21) for the health service data. Conclusion: Interestingly, we found better overall survival of diabetic men during the first years after cancer diagnosis. We hypothesize that access to health services may be advantageous to diabetic patients who are in close contact with the healthcare system.