Background Recent population-based studies in Latin American and the Caribbean (LAC) countries brought evidence of the growing burden of mental illness in this region. The objective of this study is to examine determinants of health service utilization by individuals with psychiatric disorders in a defined area in the city of São Paulo, Brazil. Method Data were derived from São Paulo Catchment Area Study (SP-ECA), a cross-sectional household prevalence survey, based on a representative adult sample (N = 1,464) living in two defined boroughs. The psychiatric diagnosis was assessed through the CIDI 1.1 interview, yielding ICD-10 diagnoses. The past-month use of health services—for general medical (GM) care and mental health (MH) care sectors—was investigated in their relationship with sociodemographic features, insurance coverage, GM conditions, and psychiatric morbidity. Results Nearly one-third (32.2%) of the total sample used health services in the last month: 29.0% attended GM care and 7.8% used MH care. Logistic regression models showed that being female, older than 60 years, having private insurance coverage, and presence of psychiatric morbidity increased the level GM care seeking in the total sample. For those with 12-month psychiatric disorders, the determinants for GM sector use were female gender, age 45–59 years old, and private insurance coverage, whereas separated, divorced, or widowed women had the highest odds (OR 9.9; 95% CI: 2.7–36.5) for using MH service. Low-income people were less likely to seek MH services. Conclusion The major contribution of this article is to underscore the impact of MH on health care systems, in a LAC country where service use information is scarce. The main finding is that inequalities in the access to MH care occurred, with low-income people having less likelihood of receiving treatment for their mental disorder. Access to health service in this catchment area reflected the great degree of deregulation and lack of integration. Additional efforts should address the barriers to the utilization of MH services in Brazil, including social inequities in the access to care.