Concurrent mental and substance use disorders or dual diagnosis are highly prevalent among individuals experiencing homelessness. Studies have indicated that dual diagnosis leads to poorer health outcomes and higher health service utilization among those affected. This study aims to estimate the prevalence of dual diagnoses among homeless populations in British Columbia (BC), Canada, and understand their characteristics and specific factors associated with dual diagnoses. The BC Health of the Homeless Survey is a cross-sectional study involving the homeless population of three cities in BC. The survey assessed addiction and concurrent disorders with standardized interviews—the Mini-International Neuropsychiatric Interview Plus, the Maudsley Addiction Profile, and the Brief Symptom Inventory—in a sample of 500 individuals who are homeless living in shelters or on the street. We characterized individuals after categorizing them into four groups: those without any current mental disorder, those with substance use disorders only, those with mental disorders only, and those with concurrent substance use and mental disorders. Focusing on the concurrent disorder group, we completed a multivariate analysis comparing individuals with dual diagnosis to those without concurrent disorders. Consistent with previous studies, we found that individuals with dual diagnoses report more severe physical and psychological symptoms. Among the homeless, they were more likely to be Aboriginal and younger and more likely to not make it into a shelter. They also reported substantially more difficulties in getting the health care service that they need. Within this marginalized group, individuals with dual diagnosis were more likely to be from groups considered to be more vulnerable with more complex needs. They were having more problems accessing even basic support, such as shelters and health care. Without a systematic approach in providing appropriate care to individuals with dual diagnosis, the most vulnerable clients are not only the ones likely to suffer the most but also the ones having the most problems meeting their basic needs.