High rates of substance use, especially cannabis and stimulant use, have been associated with homelessness, exposure to trauma, and involvement with the criminal justice system. This study explored differences in substance use (cannabis vs. stimulants) and associations with trauma and incarceration among a homeless population. Data were derived from the BC Health of the Homeless Study (BCHOHS), carried out in three cities in British Columbia, Canada. Measures included sociodemographic information, the Maudsley Addiction Profile (MAP), the Childhood Trauma Questionnaire (CTQ), and the Mini International Neuropsychiatric Interview (MINI) Plus. Stimulant users were more likely to be female (43%), using multiple substances (3.2), and engaging in survival sex (14%). Cannabis users had higher rates of lifetime psychotic disorders (32%). Among the incarcerated, cannabis users had been subjected to greater emotional neglect (p < .05) and one in two cannabis users had a history of lifetime depressive disorders (p < .05). Childhood physical abuse and Caucasian ethnicity were also associated with greater crack cocaine use. One explanation for the results is that a history of childhood abuse may lead to a developmental cascade of depressive symptoms and other psychopathology, increasing the chances of cannabis dependence and the development of psychosis.