Duncan served as treasurer and general counsel of the Republican National Committee (RNC) from July 10, 2002, until his election as chairman.

[7] Source: CQPolitics,[8] and Poll Pundit[9] President George W. Bush nominated him to the Tennessee Valley Authority Board; he was unanimously confirmed by the United States Senate in March 2006.

[13] Duncan was involved in the selection of Louis DeJoy as U.S. postmaster general amid the 2020 presidential election.

[14] After DeJoy's appointment, postal service leadership implemented measures that led to delays in mail delivery and raised concerns about voting-by-mail in the 2020 election amid the coronavirus pandemic.

[14] Duncan is on the board of the Senate Leadership Fund, which is a super PAC tied to Mitch McConnell.

They have one child, Rob, an assistant United States attorney in Lexington, Kentucky, who is married to Valerie Ridder, originally from Springfield, Missouri.

[17] On September 10, 2012, Duncan became the new president and CEO of the American Coalition for Clean Coal Electricity (ACCCE).