No research has examined the prevalence of involuntary cannabis exposure in the home within the context of multi-unit housing (MUH). The 2017 cycle of the Centre for Addiction and Mental Health Monitor population RDD survey included measures of environmental cannabis smoke (ECS) and environmental tobacco smoke (ETS) for Ontario, Canada. These ECS measures were defined for those who did not live in a detached dwelling self-reporting noticing any tobacco or cannabis smoke enter the home from a neighboring unit or from outside the building at least once in the past 6 months. Overall, 6.6% (95% CI: 4.5–9.5%) and 7.5% (9% CI: 5.4–10.4%) of the population reported being exposed to ETS and ECS in MUH respectively. Individuals exposed to ECS were single, had used cannabis in the past 12 months, and had lower household incomes. The prevalence of involuntary exposure to cannabis smoke is similar to exposure to tobacco smoke. Exposure correlates were primarily associated with characteristics of those who lived in MUH who tend to be members of more vulnerable populations.