Douglas Michael Duncan (born October 25, 1955) is a former American politician from Maryland who served as Rockville City Councilman, Rockville Mayor, Montgomery County Executive, and candidate for Governor of Maryland. He is a member of the Democratic Party. Duncan currently serves as president and CEO of Leadership Greater Washington, a position he has held since 2014.

Duncan was born on October 25, 1955. The fifth of 13 children, Duncan grew up in the Twinbrook section of Rockville, Maryland, a working-class neighborhood, home to federal employees, teachers, police officers and firefighters. His father worked for the NSA and later worked for the Montgomery County Public Schools as a volunteer tutor and ESOL teacher. His mother worked for the Montgomery County Circuit Court as a courtroom clerk from 1973 to 1999.

Duncan attended St. John's College High School in Washington, D.C., graduating in 1973. He graduated from Columbia University in three years, in 1976, earning a bachelor's degree with a double focus in psychology and political science. Upon graduating, Doug worked for Montgomery County's criminal justice commission, and then spent 13 years in the private sector working in the telecommunications industry for AT&T.

Duncan got his start in politics at an early age, going door-to-door with his mother for John F. Kennedy's presidential campaign, and was the only one of his 12 siblings to follow in his mother's Democratic-activist footsteps.

Duncan's first full-time experience in politics was as a field director for Charlie Gilchrist's campaign for Montgomery County Executive in 1978, which Gilchrist won by a margin of better than 3-to-2. Deciding to run for political office himself, Duncan ran for and was elected to Rockville City Council in 1982 at the age of 26. He would be re-elected twice to that position. In 1984, Duncan called for the resignation of then-Mayor John Freeland, who had taken a job with a city developer, claiming it was a rank conflict of interest. Freeland resigned that year.

After serving three terms as Rockville City Councilman, Duncan ran for Mayor of Rockville in 1987, beating incumbent Steven VanGrack. During his six years as Mayor, Doug started the process of redevelopment in downtown Rockville, which included tearing down the Rockville Mall. That project would serve as inspiration for his revitalization of downtown Silver Spring years later as Montgomery County Executive. Accomplishments under the Duncan administration include building Wootton Parkway and Gude Drive bridge. One of his proudest accomplishments was starting Hometown Holidays, Rockville's annual Memorial Day weekend festival.

Duncan successfully ran for Montgomery County Executive in 1994, defeating Councilman Bruce Adams by nine percentage points in the Democratic Primary and earning 63% of the vote against Steve Abrams in the general election. Duncan would go on to be re-elected twice for a then unprecedented three terms as County Executive. During Duncan's tenure as County Executive, he focused on improving educational excellence, strengthening environmental protections, fighting poverty and urban blight, and positioning Montgomery County as an international biotechnology leader and economic engine for Maryland all while managing a multi-billion dollar annual budget.

The Washington Post described his governing style as "county leadership that knows when to quit mulling and start moving." Due to his long tenure and influence, media observers later described him as "Montgomery's dominant political figure for over a decade."