Despite an aggregate 5-year survival of 85%, many adolescents and young adults (AYAs, 15–39 years old) treated for cancer die prematurely decades later. To develop a more complete understanding of this problem, particularly the role of specific subsequent malignant neoplasms (SMNs), we used the SEER-9 registry to analyze causes of death (COD: Primary cancer, SMN, non-malignant conditions) among 162,317 AYAs diagnosed with first cancer between 1975–2012 and surviving 5 or more years. Cumulative mortality, attributable mortality, standardized mortality ratios (SMRs), and adjusted hazard ratios were determined for each cancer site and COD. At 30 years, cumulative mortality due to primary cancer was matched by that due to all other causes (12.8% 95% CI [12.5%, 13.0%] for primary cancer versus 12.8% [12.5%, 13.1%] for all other causes combined) in the combined cohort, and was overtaken by non-malignant conditions in Hodgkin lymphoma, testicular, cervical/uterine, and thyroid cancers. Overall, SMNs accounted for 20% of malignant deaths, the most common being lung/bronchus (25.6%), colorectal/liver/biliary/pancreas (19.1%), and breast (10.2%). For non-malignant conditions, excess risk was noted overall (SMR 1.37, 95% CI [1.34, 1.40]) and for infectious (1.97 [1.85, 2.10]), renal (1.85 [1.60, 2.13]), cardio/cerebrovascular (1.38 [1.33, 1.43]), and suicide (1.15 [1.04, 1.27]). Racial minorities were at significantly higher risk across all COD. Safer therapy, longitudinal monitoring, and primary/secondary preventive strategies are needed to reduce late mortality in this vulnerable population.