Douglas M. Duncan (born October 25, 1955) is a Democratic politician from Maryland who served as County Executive of Montgomery County from 1994 to 2006. At three terms, Duncan held the office longer than any other County Executive in Montgomery County's history. He was a candidate for the Democratic nomination for Governor of Maryland in the 2006 elections until he abruptly dropped out of the race on June 22, 2006, citing clinical depression. Following his exit from the gubernatorial race, he was appointed Vice President of Administrative Affairs of the University of Maryland, College Park, where he worked for 17 months.
Duncan attended St. John's College High School and received a Bachelor's degree in psychology and political science from Columbia University. Duncan is married to Barbara Duncan and has five children.
Duncan was elected as Montgomery County Executive in 1994. Prior to serving as County Executive, Duncan was the Mayor of Rockville from 1987–1993 and a member of the Rockville City Council from 1982–1987. He won election to a third term as County Executive in 2002, becoming the longest serving Executive in the history of Montgomery County. Due to his long tenure and influence, media observers later described him as, "Montgomery's dominant political figure for over a decade."