He is known for his television series regular roles like Braxton P. Hartnabrig on The Jamie Foxx Show, President William Johnson in The First Family, and his portrayal of Clarence Weidman on Veronica Mars.

[citation needed] Duncan had a starring role as Braxton P. Hartnabrig on The Jamie Foxx Show.

He also had recurring roles on The Resident, Veronica Mars, Aliens in America, Lincoln Heights, Jane by Design, The District, and Soul Food.

He also guest-starred in Martin, iCarly, Rosewood, Castle, NCIS, Bones, Mental, ER, Boston Legal, 24, CSI,and The Practice.

He played Barack Obama on The Tonight Show with Jay Leno,as well as in the 2010 film My Name Is Khan.