Objectives: Despite having close contact with the general public, members of the San Francisco Fire Department (SFFD) reported relatively few cases of COVID-19 during the first half of 2020. Our objective was to explore evidence for prior undetected infections by conducting a seroprevalence survey, and to document both risk and protective factors for prior COVID-19 infection. Methods: This cross-sectional study assessed workplace practices and exposures of SFFD personnel during the first 6 months of 2020 via questionnaire and documented prior COVID-19 infections by serologic antibody testing using an orthogonal testing protocol. Results: Of 1231 participating emergency responders, three (0.25%) had confirmed positive COVID-19 antibody results. Conclusions: Safe workplace practices, community public health intervention, and low community infection rates appear to have been protective factors for emergency responders in San Francisco during our study period.