Mongan was president and CEO of Massachusetts General Hospital from 1996 to 2002, and president and CEO of Partners HealthCare from 2003 to 2009. He was instrumental in drafting the landmark Massachusetts Comprehensive Health Reform that took effect in 2006. He also served in the federal departments of Health and Human Services and Health, Education and Welfare. In 2010 Mongan received the American Hospital Association Distinguished Service Award, the association’s highest honor.