William Dunlap Simpson was born in Laurens District, South Carolina, in 1823, he was educated at South Carolina College (later the University of South Carolina), completing his studies in 1843, and spent one term at Harvard Law School. He practiced law in Laurens with his partner (and father-in-law) Henry Clinton Young, and later with his brother John Wistar Simpson (ca. 1861-1880). He served in the South Carolina legislature in the 1850s and early 1860s and served in the Confederate States House of Representatives from 1863 to 1865.  In 1868, Simpson was elected State representative to 41^st Congress, but was barred from seat.

After the Civil War, he returned to practice law in Laurens until 1876, when he ran successfully for the post of lieutenant governor and was re-elected in 1878. Upon Wade Hampton resigning from the governorship to assume his senate seat, Simpson was elevated to become the 78^th  governor of South Carolina Simpson was elevated to become the 78th governor of South Carolina on February 26, 1879. He resigned on September 1, 1880 after being appointed Chief Justice of the state Supreme Court and served for ten years from 1880 until his death in 1890.

Source: Biographical Directory of the South Carolina Senate, 1776-1985, Vol. II, 1986, pp. 1465-1466