Research has shown that individuals with autism spectrum disorder have higher rates of health problems throughout childhood, adolescence, and adulthood, and that this may result in elevated risk of early mortality. This study reported the rate, timing, and causes of death in a large community-based cohort of adolescents and adults with autism spectrum disorder (n = 406) over a 20-year period (1998-2018) and identified predictors of mortality. Over this period, 6.4% of individuals died at an average age of 39 years. Causes of death included chronic conditions (such as cancer and heart disease), accidents (such as choking on food and accidental poisoning), and health complications due to medication side effects. Even after controlling for age and health status, significant predictors of mortality were early childhood levels of impairments in social reciprocity and high levels of functional impairments at the start of the study period. The results suggest the importance of social engagement and functional self-sufficiency across the life course, as well as adequate access to health care for individuals with autism spectrum disorder.