Adverse childhood experiences (ACEs) increase risk of substance use disorders (SUDs). Little research has focused on individuals with serious mental illnesses (SMI), despite their high prevalence of both ACEs and SUDs. We combined two datasets from prior studies (n = 299 and n = 240, total n = 539) that measured ACEs and made research diagnoses for SUDs. When controlling for other variables-age, gender, race, diagnostic category (psychotic disorder versus mood disorder), and study site (Washington, DC-area versus southeast Georgia)- in logistic regression models, ACE score was associated with tobacco use, presence of any SUD, alcohol use disorder, cannabis use disorder, and cocaine use disorder. Each one-unit increase in the ACE score increased the odds of SUD-related outcomes by 9-18%. Clinicians, program planners, and researchers should be aware of the powerful and long-lasting impact of ACEs, and the need for thorough screening and assessment of both SUDs and ACEs among patients with SMI.