William Garnett Duncan (March 2, 1800 – May 25, 1875) was a United States Representative from Kentucky. He was born in Louisville, Kentucky. He completed preparatory studies and was graduated from Yale College in 1821. He studied law and was admitted to the bar in 1822 and commenced practice in Louisville, Kentucky.
Duncan was elected as a Whig to the Thirtieth Congress (March 4, 1847 – March 4, 1849) but declined to be a candidate for renomination in 1848. After leaving Congress, he moved to Louisiana and settled in New Orleans, Louisiana in 1850, where he continued the practice of law.
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.