William Garnett Duncan (March 2, 1800 – May 25, 1875) was a United States representative from Kentucky.
He studied law and was admitted to the bar in 1822 and commenced practice in Louisville, Kentucky.
Retiring from active law practice in 1860, Duncan traveled in Europe.
He resided for a while in Paris, France before returning to the United States in 1875 to Louisville, Kentucky.
He died in that city on May 25, 1875, and was buried in Cave Hill Cemetery.