Background Improved cancer survival is expected to increase noncancer deaths; however, detailed causes of death have rarely been discussed. Changing landscapes in mortality patterns and noncancer mortality risks in patients with cancer require evaluation. Methods We identified cancer and noncancer‐related causes of death using data from the 2000‐2016 national cancer registry in Korea (n = 2 707 520), and we characterized the leading causes of death and proportionate mortalities over time. Risks of noncancer deaths relative to the general population were estimated using standardized mortality ratios (SMRs). Results Of 1 105 607 identified deaths, 87% were due to the primary cancer. Proportionate mortalities of primary cancer among overall deaths remained high in patients with liver (86%) and lung (70%) cancers and in female patients with breast cancer (77%), even 5 to 10 years following diagnosis, whereas proportionate mortalities reduced to ≤50% in patients with stomach (men, 39%; women, 48%), prostate (47%), and female thyroid (27%) cancers. Despite the predominance of index cancer deaths, the proportion of noncancer deaths among all deaths increased over time. There was a 20‐fold increase in cardiovascular disease deaths among patients with cancer from 2000 to 2016, and the risk of suicide among patients with cancer was higher than that among the general population (SMR: 1.68 95% confidence interval (CI): 1.63‐1.74 in men, SMR: 1.42 95% CI: 1.33‐1.55 in women). Conclusions Deaths from primary cancer remain a major concern; however, follow‐up is required for both cancer and noncancer‐related health issues in cancer survivors, especially concerning suicide and cardiovascular deaths. Death from primary cancer is still the major concern, but noncancer deaths are expected to become an important issue. Higher risks of suicide death and increasing number of cardiovascular disease deaths in cancer patients highlight the need for preventing suicide and cardiovascular disease in cancer patients.