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.

He graduated from Columbia University in three years, in 1976, earning a bachelor's degree with a double focus in psychology and political science.

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.

[12] Duncan successfully ran for Montgomery County Executive in 1994, defeating Councilman Bruce Adams by nine percentage points in the Democratic Primary[7] and earning 63% of the vote against Steve Abrams in the general election.

Some of Duncan's top accomplishments during his three terms in office were the revitalization of downtown Silver Spring, including the relocation of Discovery's world headquarters and the opening of the AFI Silver Theatre and Culture Center, the building of Strathmore, a cultural and artistic venue and institution, the Maryland Soccer Plex, gaining the approval for building of the Intercounty Connector, and becoming the first county ever in the United States to implement an earned income tax credit.

[15] In 2005, Duncan announced his candidacy for the Democratic Party's nomination to challenge incumbent Governor Robert Ehrlich in the 2006 Maryland gubernatorial election.

He cited a family history of the disease as a factor in the diagnosis, and a number of aides and political associates were quoted in the press saying that Duncan was noticeably unhappy in the period leading up to his withdrawal.

During his withdrawal announcement, Duncan endorsed his Democratic primary opponent, Martin O'Malley, in the latter's race against incumbent Republican Governor Robert Ehrlich.

Reports at the time suggested that his dropping out would have a significant effect on state politics, making it easier for O'Malley to unseat Ehrlich and lowering turnout in the Democratic primary, thereby affecting down ballot races as well.

During his tenure, Duncan led the East Campus redevelopment project, designed as a mixed-use town center with graduate student housing along the Route 1 corridor.

[1] After Maryland, Duncan was self-employed as a consultant advising various enterprises in Greater Washington D.C. Metro Region and beyond regarding economic development, technology and immigration issues.

In 2010, Duncan was speculated as being in consideration for General Manager of WMATA, after John Catoe vacated the post, including receiving an endorsement from then Congressman, and now United States Senator Chris Van Hollen.

[22] In 2014, Duncan made his political comeback seeking to reclaim his old post at Montgomery County, Maryland Executive for a fourth term.

Running on a platform of "Leadership In Action" with ideas of revitalizing downtown Wheaton, better attract and keep jobs and business with an improved procurement process and more "open for business" attitude, create express lanes on I-270, build a technology institute venture similar to that of Cornell and New York City, and provide gigabit internet speeds across the county.