Context: Social accountability of medical schools has emerged as a standard of excellence in medical education during the last decade. However, the lack of valid and reliable instruments to estimate social accountability has limited the possibility of measuring the impact that medical schools have in society. Our aim was to develop an instrument and validate its use for assessing social accountability in Latin American countries. Methods: We used a three–phase mixed methods research design to develop, validate and estimate social accountability in a diverse convenient sample of 49 medical schools from 16 Latin American countries. We used a qualitative framework approach and a Delphi consensus method to design an instrument with high content validity. Finally, we assessed the psychometric properties of the instrument. Results: The Social Accountability Instrument for Latin America (SAIL) contained 21 items in four domains: mission and quality improvement, public policy, community engagement, and professional integrity. Its reliability index, estimated using Cronbach’s alpha, was very high (0.96). Most of the medical schools that had ranked over the 80th percentile on traditional national academic estimates did not reach the 80th percentile using SAIL. Conclusions: There are validity arguments (content and reliability) to support the measurement of social accountability using the SAIL instrument. Its application showed that it provides a complementary dimension to that traditionally obtained when estimating quality in medical schools.