•More than 90% (144/159) of inmates aged >60 were infected with SARS-CoV-2.•Only 2 hospitalisations occurred and no COVID-19 related deaths.•Key actions included periodic health monitoring, active case finding and early care. To assess SARS-CoV-2 prevalence and health outcomes among inmates over 60 years during a COVID-19 outbreak in a major penitentiary complex in the Federal District, Brazil. A mass test campaign was performed on May 13, 2020, using antibody-detection rapid tests for asymptomatic inmates and reverse transcriptase-polymerase chain reaction testing for those who were symptomatic. Those with negative results were retested on June 16. Inmates were interviewed to characterise background health conditions and the presence of symptoms. A total of 159 inmates were evaluated. In the first mass testing, 79.9% (127/159) of inmates had been infected, of whom 53.5% (68/127) reported symptoms. In the second testing round, 17 new cases were identified, increasing the total to 90.6% (144/159) of inmates with a positive result. Comorbidities were present in 67.3% of inmates; 2 hospitalisations and no COVID-related deaths were recorded. More than 90% of inmates aged >60 years were infected with SARS-CoV-2 during the outbreak. Periodic health monitoring, active case finding and early care for symptomatic patients contributed to positive post-infection outcomes. Such measures must be considered essential for the surveillance of COVID-19 in environments with limited capacity to promote social distance, such as penitentiary institutions.