Introduction The conditions of Brazilian penitentiaries are known to be affected by overcrowding and precarious cells. Violence inside the prison impacts the mental and general health of individuals in this environment, whether prisoners whether workers. Thus, an increase in cases of common mental disorders is expected among workers, which contributes for their absenteeism. Objective To assess the prevalence of common mental disorders among prison workers in the state of São Paulo, Brazil. Methods A prison unit was selected for the administration of the Self-Reporting Questionnaire-20 and a questionnaire to assess the sociodemographic profile of employees. The questionnaires were filled out in electronic format and made available on computers in the selected unit. Two visits were made to the unit to assess the work environment and to listen workers’ reports. Results Fifty-three questionnaires were selected, of which 50 were included in the research. According to the Self-Reporting Questionnaire-20, 34% of participants had positive results for CMD. The use of psychotropic drugs and less family support were statistically correlated with common mental disorders. The apparently small number of employees in the unit was considered a more harmful factor to work than personal contact with prisoners. Conclusions Although high, the prevalence of common mental disorders was lower than that described in the literature. The implementation of mental care programs for state civil servants would be essential to reduce the prevalence and absenteeism due to common mental disorders.