We examined mortality rates of adolescents and young adults before and after exiting foster care. We used administrative records to identify individuals born in California 1985-2005 who had a foster care episode on or after their 16th birthday. We linked these records to vital death records through 2022 to identify deaths occurring age 16-23. We defined three care statuses: pretransition, transition, and posttransition. Pretransition was age 16 to the last day of care. Transition was the first 120 days after exiting care. Posttransition was all days after transition. We calculated gender-standardized mortality rates (SMRs) and used a Cox proportional hazards model adjusted for demographics to estimate hazard ratios of total mortality. In total, 1,743 deaths occurred among 144,128 individuals. SMRs for pretransition, transition, and posttransition per 100,000 person-years were 116, 259, and 209, respectively. Time-varying hazards models detected that these high and disparate rates were driven by higher risk during transition and posttransition for those leaving care before age 20. Moreover, the transition period featured particularly heightened risk for those leaving care before age 18. Results suggest targeted support during the transition period could help safeguard this population from harm.