To date, only few studies have been published documenting late mortality among early onset cancer survivors, especially regarding young adulthood (YA) malignancies. Our nation-wide population-based registry study provides information concerning cause-specific long-term mortality among 16,769 5-year survivors of early onset cancer (aged 0-34 years at diagnosis), with follow-up for death extending from 1971 through 2012. A sibling cohort and population data were used as reference. The overall standardized mortality ratio (SMR) of cancer patients was 4.6-fold, (95% CI 4.4-4.8). Highest SMRs were found for malignancies (12.8, 95% CI 12.3-13.3), infectious (4.8, 95%CI 2.9-6.7) and cardiovascular diseases (1.9, 95% CI 1.7-2.1). Malignancies and cardiovascular diseases accounted for the largest number of deaths. Childhood and YA cancer survivors with the same primary cancer site had a similarly elevated overall SMR with the exception of markedly higher SMRs after childhood Hodgkin lymphoma. The highest cumulative non-malignancy-related mortality was due to cardiovascular disease with a steady rise throughout the follow-up, but strongly dependent on the primary cancer site and age at diagnosis. In childhood cancer survivors, the cumulative cardiovascular mortality did not reduce over time. However, overall and malignancy-related mortality showed a declining tendency towards the most recent periods after both, childhood and YA cancer. Our findings on non-malignancy-related mortality stress the need to set up long-term individual follow-up with a focus on cardiovascular late effects for early onset cancer survivors, especially for YA cancer survivors still lacking those.